The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - 47-55.1 Pressure bleeding

dsraven · 10-16-2021, 02:10 PM

q1
do you have a firm pedal or a spongy pedal
q2
what does your pedal linkage look like down to the master cylinder, are all the pivots and connections free moving but tight (no play) and the pedal isn't dragging on the floor seal
q3
do you have a pedal return spring and a pedal stopper at the top of the pedal stroke so the pedal has to stop at the same place every time it is released
q4
does your master cylinder have residual valves in the outlets. some rebuilt units made for disc/drum were previously used on drum/drum which had the master cyl res valves. a master made for disc/drum usually has one res bigger than the other because disc's have a bigger volume at the caliper than a drum wheel cylinder has so they need more volume in storage capacity to supply the system as the brakes wear and fluid is required. are the inline residual valves installed right way around
q 5
do the brake lines have coils , high spots etc that trap air
q6
is your linkage adjusted properly so when the pedal is at it's full released position and against the stopper there is free play on the pushrod at the brake booster end. this is required to allow the master cyl to return fully
q7
is the master cylinder to booster pushrod adjusted so the master cylinder has ability to return fully? just because the booster pushrod from the linkage has freeplay that doesn't mean the booster to master cyl pushrod is correct
q8
have you tried disconnecting a line at the master cylinder outlet and using your pressure bleeder to bleed that line to see if it is a line/system problem downstream of the master or a master cyl/booster/linkage problem? if the line bleeds easily then the problem may be that the master cylinder is simply not returning fully so the ports that allow fluid into the piston bore are only partially open to the reservoir. this is a big reason why I asked q6. lots of guys don't think a pedal stopper or return spring is needed but the weight of the pedal and linkage can easily overcome the spring at the booster so the master cylinder/booster sit in a position like somebody's foot is still on the pedal a little bit. a stopper also allows you to make the linkage free play adjustment at the booster with the pedal at the fully released position and you know that position will not change when the vehicle bumps over roads or the body twists up under load etc. the same reason a pedal return spring is also needed, to keep the pedal fully against the bump stop. if doing the bleed as suggested above and you get one line that bleeds easy and the other line doesn't then look at the prop valve, it may be shuttled if it has a metering valve integral. read on for that link
q9
do you have the park brake cables connected and adjusted properly. the park brake cables are an integral part of the rear drum brake adjustment and if not installed and adjusted properly then the system uses more fluid than normal (and thus pedal travel) to move the brakes shoes out against the drums before braking can start. the star wheel adjusts one end of the brake shoes to fit the drum and the park brake adjustment adjusts the other end of the shoes, at the wheel cylinder end, to fit the drum. otherwise the brake springs will collapse the wheel cylinders back into their bores when the brakes are released and then when you apply the brakes the first part of the pedal stroke is taken up moving fluid to those wheel cylinders to push them out until the shoes contact the drums. back in the day (well, even now) guys wouldn't bother using their park brakes or they wouldn't replace a broken or siezed cable. those were the guys that used a rock for a park brake or relied on that little park pin in the auto trans to hold their car when parked. they were also the guys that had to pump the brakes because the brake shoes collapsed back to their stoppers/anchors and it took the first brake pump to get them back out against the drums again.they would say "the brakes work fine. you just gotta pump them a bit".
q10
can you post some pics of your system
q11
is your master cylinder and prop valve all plumbed correctly to the brake system? like which port is the front brakes and which is the rear brakes? disc/drum master cylinders usually have the larger reservoir for the disc brakes so ensure that is set up correctly first. is the prop valve a combination valve that also has a built in metering valve? if so, is the metering valve portion centered? if it has the metering valve integral and the brakes were full of air the metering valve may have shuttled when you opened a bleeder screw and fluid started moving in the system. this effectively blocks the other system in the brakes so you really only get fluid to one circuit. the metering valve section will need to be re-centered to allow the brakes to operate properly. there is a tool required to properly bleed those prop valves, it keeps the prop valve open. if the valve needs to be re-centered there is a procedure for that too
https://techtalk.mpbrakes.com/propor...-bleeding-tool
mr48 is correct, don't use too much pressure on the bleeder because brake fluid in your face doesn't feel all that good when the bleeder is cracked, it takes paint off so if it gets sprayed on the paint and you don't notice it's to late, it also takes sealer off the concrete floor and dries the skin on your hands (or maybe thats the hand cleaner used to get the brake fluid off, haha). anyway, if it won't bleed with 15lbs then it sounds like your master cylinder is not fully returning or the prop valve has shuttled. re-center the prop valve and block it open and try disconnecting the pedal linkage and see if it bleeds then. if it does then adjust the pedal linkage as it may be holding the master from fully returning.
if you have a helper, and some safety glasses, and some room to see into the master cyl reservoir, have the helper step on the brake pedal while you look into the reservoir. you should see a couple of little fountains of fluid in the reservoir when the pedal is first stepped on. that is because the "feeder hole" for the bore is uncovered which allows fluid to fill the bore and also return to the res after a brake application or when the brakes heat up and the fluid expands. when you step on the brakes that very first little bit of pedal travel causes some fluid to be sqirted back into the res until the piston passes the feeder hole. if you don't see that little fountain or disturbance in the res fluid then possibly the master cyl is not fully retracting so the piston is partially covering that feeder hole to start with which would make it real hard to bleed the brakes and also cause brake operational issues. the piston in the bore needs to be able to retract fully when released so it sits behind that feeder hole. another reason for the pedal stopper and return spring, so the pedal stops at the same place every time it is released. figure out your pedal stopper and return spring, then figure out your pushrod adjustment between the master cylinder and booster so there is the right amount of free play, then figure out your linkage adjustment between the pedal and the booster so you have free play there. ensure you have the drum brakes adjusted properly with operational park brakes, ensure your brake lines don't have air traps in them, check to ensure you aren't doubling up on residual valves, ensure your prop valve is centered, then try bleeding the brakes. if you think the master to booster pushrod is possibly the culprit, and you just wanna do a quick check without removing the whole thing, you can try installing a flat washer on each mounting stud for the master cyl. between the master and the booster. this will give you some free play in that pushrod and if that fixes the problem then you know the pushrod is not adjusted properly

drum brake adjustment, assuming you are starting with clean parts that aren't gummed up with brake dust, mud etc and all the parts are operation, the backing plates do not have grooves where the brake shoes rub, all the springs are installed where they need to be, the star wheel adjuster is free to turn and lubricated, the drums aren't grooved out, the wheel cylinders operate smoothly etc etc. ensure you have the wheels on and tightened up as this makes sure the drums are fully seated on the axle flanges
-loosen park brake adjustment fully. this allows the wheel cylinder end of the shoes to fully retract against the anchor
-tighten the star wheel fully. not just until the brakes drag a little as that doesn't mean the shoes are actually centered in the drums. crank the star wheel until you physically can't turn the wheel
-back the star wheel off until the brakes drag just a slight bit, the tire is easy to turn. now the one end of the shoes is adjusted to fit the drum diameter
-now adjust the park brake. this will adjust the other end of the brake shoes to fit the drum diameter. now when the brakes are pushed the wheel cylinders will be pushed out and held at that spot when released. they will take less fluid on the next application so the pedal stroke will be less. no more "pedal goes to floor"
-understand that if you have new brake shoes and drums it doesn't mean they have the same diameter and will fit perfectly right away. that is the reason the park brake seems to need adjustment not long after your new brake job. the first bit of driving wears the shoe friction material to fit the drum diameter. after doing a brake job on a vehicle with drum brakes if you find the brake pedal height changes a bit you may need to do a few stops in reverse so the auto adjusters will do their job, then adjust the park brake again. if you had the drums machined then they are a larger diameter than the shoes so unless you bought the oversized shoes and had them re-arched to fit the drums ( not a lot of shops can even do that anymore, they just sell oversized shoes or new drums and regular shoes) then it may take some time to wear the new shoes to fit the oversized drums. dunno if that makes sense to you?
rear brakes should look like this. ensure the longer lining is in the right spot, the star wheel is the right way around so the auto adjuster works, etc
https://www.classicmuscle.com/p-75-8...hields-dbk75r2

this shows the residual valve locations in the master cylinder along with a few other topics and diagrams of how stuff works
https://slidetodoc.com/chapter-29-hy...brake-systems/

hope you get it working. let us know if you need more support.

10-16-2021, 02:10 PM	#5
dsraven Registered User Join Date: Nov 2010 Location: calgary alberta Posts: 7,833	Re: Pressure bleeding q1 do you have a firm pedal or a spongy pedal q2 what does your pedal linkage look like down to the master cylinder, are all the pivots and connections free moving but tight (no play) and the pedal isn't dragging on the floor seal q3 do you have a pedal return spring and a pedal stopper at the top of the pedal stroke so the pedal has to stop at the same place every time it is released q4 does your master cylinder have residual valves in the outlets. some rebuilt units made for disc/drum were previously used on drum/drum which had the master cyl res valves. a master made for disc/drum usually has one res bigger than the other because disc's have a bigger volume at the caliper than a drum wheel cylinder has so they need more volume in storage capacity to supply the system as the brakes wear and fluid is required. are the inline residual valves installed right way around q 5 do the brake lines have coils , high spots etc that trap air q6 is your linkage adjusted properly so when the pedal is at it's full released position and against the stopper there is free play on the pushrod at the brake booster end. this is required to allow the master cyl to return fully q7 is the master cylinder to booster pushrod adjusted so the master cylinder has ability to return fully? just because the booster pushrod from the linkage has freeplay that doesn't mean the booster to master cyl pushrod is correct q8 have you tried disconnecting a line at the master cylinder outlet and using your pressure bleeder to bleed that line to see if it is a line/system problem downstream of the master or a master cyl/booster/linkage problem? if the line bleeds easily then the problem may be that the master cylinder is simply not returning fully so the ports that allow fluid into the piston bore are only partially open to the reservoir. this is a big reason why I asked q6. lots of guys don't think a pedal stopper or return spring is needed but the weight of the pedal and linkage can easily overcome the spring at the booster so the master cylinder/booster sit in a position like somebody's foot is still on the pedal a little bit. a stopper also allows you to make the linkage free play adjustment at the booster with the pedal at the fully released position and you know that position will not change when the vehicle bumps over roads or the body twists up under load etc. the same reason a pedal return spring is also needed, to keep the pedal fully against the bump stop. if doing the bleed as suggested above and you get one line that bleeds easy and the other line doesn't then look at the prop valve, it may be shuttled if it has a metering valve integral. read on for that link q9 do you have the park brake cables connected and adjusted properly. the park brake cables are an integral part of the rear drum brake adjustment and if not installed and adjusted properly then the system uses more fluid than normal (and thus pedal travel) to move the brakes shoes out against the drums before braking can start. the star wheel adjusts one end of the brake shoes to fit the drum and the park brake adjustment adjusts the other end of the shoes, at the wheel cylinder end, to fit the drum. otherwise the brake springs will collapse the wheel cylinders back into their bores when the brakes are released and then when you apply the brakes the first part of the pedal stroke is taken up moving fluid to those wheel cylinders to push them out until the shoes contact the drums. back in the day (well, even now) guys wouldn't bother using their park brakes or they wouldn't replace a broken or siezed cable. those were the guys that used a rock for a park brake or relied on that little park pin in the auto trans to hold their car when parked. they were also the guys that had to pump the brakes because the brake shoes collapsed back to their stoppers/anchors and it took the first brake pump to get them back out against the drums again.they would say "the brakes work fine. you just gotta pump them a bit". q10 can you post some pics of your system q11 is your master cylinder and prop valve all plumbed correctly to the brake system? like which port is the front brakes and which is the rear brakes? disc/drum master cylinders usually have the larger reservoir for the disc brakes so ensure that is set up correctly first. is the prop valve a combination valve that also has a built in metering valve? if so, is the metering valve portion centered? if it has the metering valve integral and the brakes were full of air the metering valve may have shuttled when you opened a bleeder screw and fluid started moving in the system. this effectively blocks the other system in the brakes so you really only get fluid to one circuit. the metering valve section will need to be re-centered to allow the brakes to operate properly. there is a tool required to properly bleed those prop valves, it keeps the prop valve open. if the valve needs to be re-centered there is a procedure for that too https://techtalk.mpbrakes.com/propor...-bleeding-tool mr48 is correct, don't use too much pressure on the bleeder because brake fluid in your face doesn't feel all that good when the bleeder is cracked, it takes paint off so if it gets sprayed on the paint and you don't notice it's to late, it also takes sealer off the concrete floor and dries the skin on your hands (or maybe thats the hand cleaner used to get the brake fluid off, haha). anyway, if it won't bleed with 15lbs then it sounds like your master cylinder is not fully returning or the prop valve has shuttled. re-center the prop valve and block it open and try disconnecting the pedal linkage and see if it bleeds then. if it does then adjust the pedal linkage as it may be holding the master from fully returning. if you have a helper, and some safety glasses, and some room to see into the master cyl reservoir, have the helper step on the brake pedal while you look into the reservoir. you should see a couple of little fountains of fluid in the reservoir when the pedal is first stepped on. that is because the "feeder hole" for the bore is uncovered which allows fluid to fill the bore and also return to the res after a brake application or when the brakes heat up and the fluid expands. when you step on the brakes that very first little bit of pedal travel causes some fluid to be sqirted back into the res until the piston passes the feeder hole. if you don't see that little fountain or disturbance in the res fluid then possibly the master cyl is not fully retracting so the piston is partially covering that feeder hole to start with which would make it real hard to bleed the brakes and also cause brake operational issues. the piston in the bore needs to be able to retract fully when released so it sits behind that feeder hole. another reason for the pedal stopper and return spring, so the pedal stops at the same place every time it is released. figure out your pedal stopper and return spring, then figure out your pushrod adjustment between the master cylinder and booster so there is the right amount of free play, then figure out your linkage adjustment between the pedal and the booster so you have free play there. ensure you have the drum brakes adjusted properly with operational park brakes, ensure your brake lines don't have air traps in them, check to ensure you aren't doubling up on residual valves, ensure your prop valve is centered, then try bleeding the brakes. if you think the master to booster pushrod is possibly the culprit, and you just wanna do a quick check without removing the whole thing, you can try installing a flat washer on each mounting stud for the master cyl. between the master and the booster. this will give you some free play in that pushrod and if that fixes the problem then you know the pushrod is not adjusted properly drum brake adjustment, assuming you are starting with clean parts that aren't gummed up with brake dust, mud etc and all the parts are operation, the backing plates do not have grooves where the brake shoes rub, all the springs are installed where they need to be, the star wheel adjuster is free to turn and lubricated, the drums aren't grooved out, the wheel cylinders operate smoothly etc etc. ensure you have the wheels on and tightened up as this makes sure the drums are fully seated on the axle flanges -loosen park brake adjustment fully. this allows the wheel cylinder end of the shoes to fully retract against the anchor -tighten the star wheel fully. not just until the brakes drag a little as that doesn't mean the shoes are actually centered in the drums. crank the star wheel until you physically can't turn the wheel -back the star wheel off until the brakes drag just a slight bit, the tire is easy to turn. now the one end of the shoes is adjusted to fit the drum diameter -now adjust the park brake. this will adjust the other end of the brake shoes to fit the drum diameter. now when the brakes are pushed the wheel cylinders will be pushed out and held at that spot when released. they will take less fluid on the next application so the pedal stroke will be less. no more "pedal goes to floor" -understand that if you have new brake shoes and drums it doesn't mean they have the same diameter and will fit perfectly right away. that is the reason the park brake seems to need adjustment not long after your new brake job. the first bit of driving wears the shoe friction material to fit the drum diameter. after doing a brake job on a vehicle with drum brakes if you find the brake pedal height changes a bit you may need to do a few stops in reverse so the auto adjusters will do their job, then adjust the park brake again. if you had the drums machined then they are a larger diameter than the shoes so unless you bought the oversized shoes and had them re-arched to fit the drums ( not a lot of shops can even do that anymore, they just sell oversized shoes or new drums and regular shoes) then it may take some time to wear the new shoes to fit the oversized drums. dunno if that makes sense to you? rear brakes should look like this. ensure the longer lining is in the right spot, the star wheel is the right way around so the auto adjuster works, etc https://www.classicmuscle.com/p-75-8...hields-dbk75r2 this shows the residual valve locations in the master cylinder along with a few other topics and diagrams of how stuff works https://slidetodoc.com/chapter-29-hy...brake-systems/ hope you get it working. let us know if you need more support.