brake booster question

[tommys72]

Can someone school me on my booster please? Single or dual diaphragm? Anything else pertinent? Im thinking its either gone bad or going bad. Pedal is a little mushy, wants to go with the brake pressed at a stoplight. And the rear cavity (closest to firewall) was real low tonight, but have no leaks anywhere in the brake system. No leaks to the farage floor at least.
Thanks
Tom

[Lowenuf]

more than likely your rear seal is out of your master cylinder and leaking by. You won't see any fluid because the front seal in the booster is deteriorated and the fluid is going into the booster. I just went through the same issue 2 weeks ago...

I got my booster through NAPA, and master cylinder from AutoZone.....

[tommys72]

Whatd you have to fix or replace? Were you missing the fluid also?

Thanks for the answer also. If i pull the master cylinder will i see brake fluid in the booster puddling?

[Andy4639]

To check the master just un bolt it and slide it away from the booster. Look and see if you see any sign of fluid in the back of the MC are in the booster itself. If so the booster maybe bad also. The fluid is sucking up into the booster.


This from CPP not me,
Test Power Brake Booster

If the pedal feels "hard" while the engine is running, the booster isn't operating correctly. If you suspect the booster is defective, do not attempt to disassemble or repair the power booster. Doing so is unsafe and will void your warranty.

Test 1
1.With the engine off, pump the brake pedal to remove any residual vacuum in the booster.
2.Hold pressure on the pedal while you start the engine. When the engine starts, the pedal should drop about a 1/4", this indicates that the booster is working properly.

Test 2
1.Run the engine a couple of minutes.
2.Turn the engine off and press the pedal several times slowly. The first pump should be fairly low. The second and third should become slightly firmer. This indicates an airtight booster.

Test 3
1.Start the engine and press the brake pedal, then stop the engine with the pedal still pressed. If the pedal does not drop after holding the pressure on the pedal for 30 seconds, the booster is airtight.

Inspect the Check Valve
1.Disconnect the vacuum hose where it connects to the intake manifold. Do not disconnect the vacuum line from the booster. Air should not flow when pressure is applied, but should flow when suction is applied. If air flows in both directions or there is no air flow, the valve needs to be replaced.

Verify Enough Vacuum
1.Check the operating vacuum pressure when the engine is at normal operating temperature. There should be a minimum of 18 in. of vacuum. Vacuum may be increased by properly tuning the engine, checking for vacuum leaks and blockages in vacuum lines.

[tommys72]

Thanks Andy. Cpp's tests 1-3 tell me the booster is working correctly, need to check the check valve, and check for enough vaccum to be sure. Ill check the master cyl this weekend for a leak. Pretty sure the booster is good now though

[Andy4639]

Originally Posted by tommys72

Thanks Andy. Cpp's tests 1-3 tell me the booster is working correctly, need to check the check valve, and check for enough vaccum to be sure. Ill check the master cyl this weekend for a leak. Pretty sure the booster is good now though

[Mike_The_Grad]

If you have a brake pedal that slowly drops towards the floor while you are at a stoplight, then that is your master cylinder leaking somewhere and is unable to maintain pressure to the brakes.
What year/model is your truck? Drum/drum or disc/drum setup?

But to answer your questions based on your photos, Im positive that you have a "Delco Morraine" brake booster and what looks like a disc/drum master cylinder.

Even if you do not find brake fluid leaking from the rear of the master cylinder where it mounts to the brake booster and you haven't noticed any brake fluid on the floor of your garage, does not mean that you don't have a leak.
What I mean is that the master cylinder can develop a situation where it has an "internal leak". In a dual resevoir master cylinder, there are two pistons that displace brake fluid through out the hydraulic brake system on demand. They are directly inline with each other, one for each reservoir. When operating correctly, each piston displaces brake fluid proportionately to their input,(some biasing takes place to prevent premature application of the front brakes and over application of the rear brakes.) The pistons then return to their respective positions once the driver releases the brake pedal. There are restrictive orifices inaide the master cylinder to compensate for each pistons movement.(compensation ports)Any fluid lost is replaced at this time from whichever reservoir corresponds to the leaking part of the system.(front/back half)
Now, if things do not function correctly, due to an external fluid leak or rupturing of the piston seals inside the master cylinder bore for instance, then the fluid displaced by each piston is disproportionate. External fluid leaks would be evident by brake fluid leaks somewhere visible, but internal pressure leaks are identified by the driver having to exert more effort and continually so as to compensate for the pistons seal leaking the pressure needed to effectively apply the brakes.
You posted that the rear reservoirs level has seemed to be lower than the front. If you have a 1/2 ton then the rear reservoir corresponds to the rear brakes, if you have a 3/4 ton then the rear reservoir corresponds to the front brakes (it differs with year too, I think...)
If you don't notice any fluid leaking from the rear of the master cylinder when you separate it from the brake booster, then you have to check the entire length of brake line all the way to the rear wheel cylinders.
I'm guessing your master cylinder is bad. You can perform the tests like the other posts say on the booster to know for sure, but I think your booster might be alright. If it turns out that the master cylinder is bad, then I highly recommend that you replace it with a NEW unit, not a remanufactured unit.(I think they only cost $30...could be different in your area.) And I highly recommend that you replace the rubber brake lines at the same time.(2 for the front and 1 for the rear.)
It'll save you from having to bleed the brake system twice or like in my case 3 times...lol.
Also, you must bench bleed the new master cylinder, it's the only way to purge all of the air out of the master cylinder for certain.

I just replaced my entire brake system over the course of a year. I'm only trying to save someone else the headache and work of doing things twice if possible.

[tommys72]

Thank you for that Mike. Its a 72 1/2 ton cheyenne super, disks/drums. I believe the booster is good after a few tests. So im thinking i need to take it off the booster and have a look.

The rear reservoir was pretty low, almost seemed the front was fuller than should be. The rubber "cone" in the front reservoir was squished up too.

[Mike_The_Grad]

The level of each reservoir should be within 1/4" from full. The rubber diaphragm in the cover is meant to expand when the fluid level drops, it's to displace the fluid lost, otherwise there would be a vacuum created, or air would find its way into the system.
All the symptoms you describe indicate a leak in the rear half of your brake system. So it's either in the master cylinder itself or somewhere between the proportioning valve and the rear wheel cylinders.
Remember brake fluid removes paint, and brake fluid does not evaporate. So you could start your search by grabbing some degreaser or brake cleaner and start hitting the brake lines, I'd start at the master cylinder and work my way towards the rear cleaning the lines as you go, there are a few threaded unions along the way. I think one at each frame rail in the front close to the main crossmember, and one or two towards the rear of the vehicle, don't rule out possible weak spots in the brake line tubing.
I had a leak about 2 " above a threaded union on the driver side frame rail. I guess years ago my header primary tubes were rubbing alongside the tubing. And it happened to be at a bend so it wore through the metal brake line over time. Also the union on the passenger side of my truck was slightly loose. I don't know how, but it happend.
When's the last time you inspected your rear drum brakes? Or serviced them?

[tommys72]

Just did drums and shoes not long ago, everything looked good and i replaced a brake line too. Didnt see a leak anywhere. So my rear diaphragm was fully expanded in the really low cavity, the front diaphragm was squished up in what looked like an overfull cavity. So that is correct from what you said. Ill get a look at the rear brake lines, but im betting its the master cylinder gone bad.

[Mike_The_Grad]

That's correct about the diaphragm. It's easy to eliminate an external master cylinder leak, just wipe the bottom with a clean cloth, should be bone dry, unscrew the two bolts that connect it to the booster and slide it forward an inch or two, should be bone dry. Don't fool yourself into thinking it's "moisture" or "condensation" that stuff will evaporate. Brake fluid will not.

[Mike_The_Grad]

Also the line you replaced, was it a prefab bed replacement line? Or did you bend and flare it yourself? Did you double flare it, if you did fab it yourself? I only ask because these are things I thought I had done correctly, but i didnt. Easy fix though, just a little tighter on the flaring tool, or a little more bend in the line to help it seat properly.

[tommys72]

It was a pre made line, i just put a few bends in it to get it to fit where i needed. Its all been good for quite a while now u til a few days ago when i noticed this stuff starting to happen

[Mike_The_Grad]

Try not to let either reservoir run completely dry. If it does, you will have to remove the master cylinder and bench bleed it.
By the looks of one of your original pictures, it's probably a good time to replace that vacuum line and check valve,along with the rubber grommet that goes to the brake booster. Probably cost you less than $10 for a new valve, grommet and a length of vacuum line.

[Mike_The_Grad]

I actually got an upgraded master cylinder for my setup.(larger resevoirs) Cost me a whole $37. The brake booster on the other hand was like $100, with a $35 core charge. Here's a pic.

[tommys72]

Think im going to replace the master cyl, along with the check valve, vac line, and grommet. I couldnt find anything that even resembled a brake fluid leak anywhere on the rear of the truck....only thing leaking out there this morning was my dang nose! It was cold out there

[Mike_The_Grad]

Lol. IMO I'd verify that it's actually the master cylinder that's leaking. Like I posted before, its pretty simple to confirm. Just remove the two nuts that secure the MC to the brake booster. You will see a rubber grommet and the brake booster pushrod sticking out. I'd say if you find brake fluid there it's indeed the master cylinder leaking. But that also means that all the fluid in the reservoir has gone into the brake booster, plus how ever much you've added along the way to keep it topped off. Another check you can do is to remove that vacuum check valve from the booster and stick something like a pipe cleaner (make sure it's not sharp at the end) down inside the brake booster, and if it comes up damp then I'd try to remove e the fluid somehow, especially if it's a lot. There shouldn't be any type of moisture in there. But on the plus side, if you find that the booster gives out later down the road, it's a pretty easy part to replace, if you plan ahead and find a way to support the master cylinder, you don't even have to mess with the brake system. Two nuts on the front, and four on the back. If it's possible, take the master cylinder in with you to match it up with a replacement. There are a few different ones that look somewhat similar. Or as in my case, the new one I got is completely different in shape and size, but is the correct one for my application.
Also like I mentioned before, now would be a good time to replace the rubber brake lines, since your gonna have to bleed the entire system anyways. Any signs of cracking, soft or hard spots in the rubber line is grounds for replacement.
Make sure you don't do as I did and not buy enough brake fluid. I bought a quart which is approx. How much the system holds, but the fluid coming out of the bleeders was nasty stuff. And I needed another quart and then some in order to flush the lines until clear new fluid came out and to top of the master cylinder.
I hate bleeding brakes...lol.
Don't forget to hold in the prop valve "centering pin" on the end of the valve. It's located under a little round rubber boot at the front end of the prop valve.
You must keep the pin depressed while bleeding the brake system or your brake system warning light will turn on inside the dash cluster, and they can be a little tricky to recenter. This is all assuming your gonna under take the task yourself. If your gonna have a shop do it, by all means, let them have at it!

[tommys72]

Yea, ill do it myself. Probably get to messing with it on monday when its warm again

[tommys72]

Took the master cyl off the booster and found this....think i may have found the issue? Leaking master cyl?

[Mike_The_Grad]

Bingo.
It looks like it's been leaking for a while. Remove the check valve from the vacuum line and look for brake fluid residue inside the rubber line This might give you an idea of how long and how much the MC has been leaking.
If no sign of fluid In the line that's good news, because the MC probably has a small slow leak and may have not been enough to damage the diaphragm inside the booster.
If there is residue in the line, try and shine a flashlight into the brake booster through the hole for the check valve. Hopefully it's not to bad in there, if you notice rust, scale, or any crap inside the booster, then chances are it'll fail sometime in the near future. Don't spray any cleaner or degreaser inside the booster. It'll only damage the rubber diaphragm inside there. If it does fail, it will be very obvious, very quickly. The amount of foot pressure to apply the brakes will increase a lot. You will still have full braking power to the wheels though. Also you will notice the engine running different. When that diaphragm ruptures it creates a pretty big vacuum leak, and your engine will idle and run differently. You may even be able to hear it sucking air continuously.

[tommys72]

Appreciate all the info Mike. Got lucky and found a pipe cleaner in my daughter's craft box. No brake fluid in the bottom of the booster. I dont know how long its been leaking. Its either not a big leak or been leaking long. Honestly i just noticed it feeling a little funny a few days ago and asked about it 2 days ago

[RichardJ]

>>each piston displaces brake fluid proportionately to their input,(some biasing takes place to prevent premature application of the front brakes and over application of the rear brakes.)<<

Neither is true. Brake pressure from the front and rear port are the same as long as there is no failure in the MC or any other part of the brake system.

>>You must keep the pin depressed while bleeding the brake system or your brake system warning light will turn on inside the dash cluster<<

Also not true. The pin is the end of the Metering Valve that is housed inside The Combination Valve. The Warning Switch operates independently from the Metering valve.
The Metering Valve HOLDS OFF fluid from the front brakes until line pressure reach's 40-60 lbs, depending on application. This holds off application of the front brakes until the rear brakes can begin to work and prevents nose-diving of the front end.
The Warning Switch Piston does not move if the pressure on both inlet ports is the same. 100lb on the front and 100lb on the rear port, the piston does not move. 600lb on the front port and 600lb on the rear port and the piston does not move. Sometimes there are springs on the front and rear of the switch piston so there can be some variation from equal pressure before switch contact is made and the light comes on.

[tommys72]

More good info. Thanks Richard

[Mike_The_Grad]

Originally Posted by tommys72

Appreciate all the info Mike. Got lucky and found a pipe cleaner in my daughter's craft box. No brake fluid in the bottom of the booster. I dont know how long its been leaking. Its either not a big leak or been leaking long. Honestly i just noticed it feeling a little funny a few days ago and asked about it 2 days ago

No problem. That's what I love about this website. People helping people.
It's funny you mention about the pipe cleaner, I also borrowed a few from my girlfriend's kid arts and crafts stuff. lol. Popsicle sticks work great for scraping and smearing stuff too.

[Mike_The_Grad]

Originally Posted by RichardJ

>>each piston displaces brake fluid proportionately to their input,(some biasing takes place to prevent premature application of the front brakes and over application of the rear brakes.)<<

Neither is true.

>>You must keep the pin depressed while bleeding the brake system or your brake system warning light will turn on inside the dash cluster<<

Also not true. The pin is the end of the Metering Valve that is housed inside The Combination Valve. The Warning Switch operates independently from the Metering valve.
The Metering Valve HOLDS OFF fluid from the front brakes until line pressure reach's 40-60 lbs, depending on application. This holds off application of the front brakes until the rear brakes can begin to work and prevents nose-diving of the front end.
The Warning Switch Piston does not move if the pressure on both inlet ports is the same. 100lb on the front and 100lb on the rear port, the piston does not move. 600lb on the front port and 600lb on the rear port and the piston does not move. Sometimes there are springs on the front and rear of the switch piston so there can be some variation from equal pressure before switch contact is made and the light comes on.

"Brake pressure from the front and rear port are the same as long as there is no failure in the MC or any other part of the brake system."
True statement.

each piston displaces brake fluid proportionately to their input.
Also a true statement.(layman's terms)


The Metering Valve HOLDS OFF fluid from the front brakes until line pressure reach's 40-60 lbs, depending on application. This holds off application of the front brakes until the rear brakes can begin to work and prevents nose-diving of the front end.
True statement.

some biasing takes place to prevent premature application of the front brakes and over application of the rear brakes.
Also a true statement.(layman's terms)

The warning Switch Piston does not move if the pressure on both inlet ports is the same.
True statement.


>>You must keep the pin depressed while bleeding the brake system or your brake system warning light will turn on inside the dash cluster<<

I apologize for this statement, as it needs some clarification.

"The pin" that is "depressed" in actuality is a preventative precaution. And is not so much "depressing" it, but rather preventing the "warning piston" from becoming un-centered during the brake system bleeding process.
If air is present between the outlets of the MC and the inlet ports of the "combination valve" this can result in an un-equal pressure variance between the front and rear brake systems.
It is possible that no air exists in this section,which would allow you to successfully bleed the brake system without an un-equal pressure variance between the front and rear brake systems.

But, how likely is that?

The factory service manual instructs the technician to take this precaution while bleeding the brake system, so as to avoid the added time of re-centering the warning piston when it could have been prevented in the first place.