68 Brake Help can't figure out what I missed

[Unofornaio]

HERE ARE THE SYMPTOMS:
1: PRESSING the pedal produces a hissing sound (clearly coming from booster)
2: The vac gauge drops in concert with the hissing.
3: When pumping the brakes the pedal gets hard and STAYS hard till I release it.
4: The initial push (as if to stop) of the pedal goes to the floor with the same resistance characteristic of air in the lines, some resistance but pedal is loose and goes to the floor if I push it there.
5: There are NO visable brake fluid leaks.
6: There has been NO LOSS of brake fluid from the resivor.
7: All lines are clear and flowing.
8: Pumping brakes till hard while engine off then starting makes no change pedal still hard.
I was going to remove the master bolts and peak at the cylinder just to see if there is fluid but again I have had NO loss of fluid. Meaning the resivor level has not dropped.
One more thing the brake set up and components are all the stock components nothing has been changed or modified.
Single pot master firewall mounted booster and air resivor under driver side behind running board/step.

Thank you

[siggyfreud]

To me it simply sounds like there is air in the brake system somewhere. The hissing could be from the ability to compress the pedal so far so fast, and the air being moved within the booster.

If you didn't bench bleed the master you might try that, and then rebleed the system. Are you vacuum bleeding, or doing the 2-person pedal push method?

[RustyPile]

You didn't say what parts have been replaced.. Have you replaced anything?? Pedal going to the floor (without pumping) is a sign of either a bad master cylinder or air in the system.. Leaking/loosing fluid is NOT the only possible master cylinder failure.. Just to reiterate - You must thoroughly bench bleed a new master cylinder before installing it. Once it's installed, you still have to bleed the rest of the system..

Air is really good at finding places to "hide".. I replaced all hydraulic components of the brake system on my '71, including converting to dual diaphragm booster.. I went through almost a quart of fluid bleeding the system..

[Wrenchbender Ret]

You have air in the system or you have excessive clearance between the shoes & drums. Adjust the brakes on all the wheels till the shoes are close to the drums.
George

[Unofornaio]

Originally Posted by Wrenchbender Ret

You have air in the system or you have excessive clearance between the shoes & drums. Adjust the brakes on all the wheels till the shoes are close to the drums.
George

Thank you. I had already adjusted the brakes all the way around. Till I could hear them touch the back off a couple of clicks.

What started this process was a failed wheel cylinder in the rear. Turned out all but one we're bad. All the wheel cylinders were replaced my rears have 2 per side fronts one.

I flushed out the entire system till I got new fluid. Front two flex lines were gunked up took them off cleaned up and replaced them. Verified they were flowing properly.
Prior to connecting anything back together I made sure there was clean fresh fluid in all lines and that it was flowing properly ( not restricted from line blockage)
All the shoes and drums we're in perfect condition and with in .038 of new specs.
During the process there is the possibility the master my have run out of fluid. I say may because I had run from the back to the front when letting the fluid flow to flush and saw the resivor down. I didn't look to see if it was the whole way or not just poured some back in. So there is the possibility it may need bled.

I'm using the one man and a stick method to bleed them never had trouble before with it.

Thank you all for your help.
I'm going to have to find the fittings to bleed the MC. I know it's a case of great divide but is there any validity to the theory of bleeding it on the vehicle? Not trying to be lazy just curious.

[RichardJ]

>> I had already adjusted the brakes all the way around. Till I could hear them touch the back off a couple of clicks.<<

You heard them touch?????
Adjust shoes until they drag when rotating the wheel. Step on brake pedal to center the shoes and readjust till only a slight and normally intermittent drag is felt. No need to back off, but step on pedal to recheck drag.

>>Single pot master firewall mounted booster and air reservoir under driver side behind running board/step.<<

"single pot" ? master ? What year is this truck, or do you mean single diaphragm booster?
"air reservoir under driver" what truck is this?
Have you checked air/vacuum hoses for leaks?

>>8: Pumping brakes till hard while engine off then starting makes no change pedal still hard. <<

Under those conditions, the pedal should drop when starting engine. Either the booster is bad or the vacuum source is insufficient.

There is a possibility of more than one problem.

[Unofornaio]

Originally Posted by RichardJ

>> I had already adjusted the brakes all the way around. Till I could hear them touch the back off a couple of clicks.<<

You heard them touch?????
----I adjusted them till as you said slight and intermittent drag was felt. BUT I did not step on pedal to center. The rears have dual cylinders and 2 adjusters fronts have only one.

Adjust shoes until they drag when rotating the wheel. Step on brake pedal to center the shoes and readjust till only a slight and normally intermittent drag is felt. No need to back off, but step on pedal to recheck drag.

>>Single pot master firewall mounted booster and air reservoir under driver side behind running board/step.<<

"single pot" ?
----It has a single resivor master cylinder.

What year is this truck,
---This is a 68 C40

or do you mean single diaphragm booster?
---Not sure if it's single or double but I have the original part number I will look it up.

"air reservoir under driver" what truck is this?
----These trucks had a reserve aux tank under the cab to supplement air over hydraulic trailer towing.
Have you checked air/vacuum hoses for leaks?
----There are no other air/ vac leaks that I can hear or locate.

>>8: Pumping brakes till hard while engine off then starting makes no change pedal still hard. <<

Under those conditions, the pedal should drop when starting engine. Either the booster is bad or the vacuum source is insufficient.
--vac comes from the intake manifold. With it connected as it should be and I'm getting about 19-20 on the dash guage. As the truck sits off right now it's holding at 20. If the diaphragm was bad wouldn't I loose pressure as it sits?

----I suspected booster all along but just was not sure if the brakes being out of adjustment or air in the system would ACT like booster Issue. Would this be the case?

There is a possibility of more than one problem.

Thank you. I agree there could be more than one problem. My main question now is. Would the adjustment and or air in the lines make the booster act this way? Or is it safe to assume the booster being bad is just one of the problems? The reason I'm still holding out hope it's not the booster is it did not behave like this prior to the work I did.

[Steeveedee]

Originally Posted by Unofornaio

HERE ARE THE SYMPTOMS:
1: PRESSING the pedal produces a hissing sound (clearly coming from booster)
2: The vac gauge drops in concert with the hissing.
3: When pumping the brakes the pedal gets hard and STAYS hard till I release it.
4: The initial push (as if to stop) of the pedal goes to the floor with the same resistance characteristic of air in the lines, some resistance but pedal is loose and goes to the floor if I push it there.
5: There are NO visable brake fluid leaks.
6: There has been NO LOSS of brake fluid from the resivor.
7: All lines are clear and flowing.
8: Pumping brakes till hard while engine off then starting makes no change pedal still hard.
I was going to remove the master bolts and peak at the cylinder just to see if there is fluid but again I have had NO loss of fluid. Meaning the resivor level has not dropped.
One more thing the brake set up and components are all the stock components nothing has been changed or modified.
Single pot master firewall mounted booster and air resivor under driver side behind running board/step.

Thank you

#3 There means that you have the hydraulics done, since you can get a hard pedal. But you need to take the rest of the adjustment up on your shoes.

[RichardJ]

>> it did not behave like this prior to the work I did. <<

That is key, before you look anywhere else. Going by your description alone, I have to question the brake shoe adjustment.

[Unofornaio]

Thank you both. So, and I'm not questioning your advice, but just to clarify I'm understanding correctly. Doing these adjustments MAY solve the vacuum problem? With the truck off and pressing the pedal vacuum return to 0.
I just got off the phone with a local brake Shopp that's been in business here for about 30 years ( not that that equates to knowing what they are talking about) but he told me it's the booster...the 300.00 booster he wants to sell me of course. Lol
At any rate I'm done till Monday I do the mobile food Friday- Sunday and have to prep all that fun stuff. I will get back to it on Monday.
Thank you again I really appreciate it. I've been pulling what little hair I have left out.

[Steeveedee]

Oh, and by the way, those dual-plane brakes can be tricky to adjust. It may take a few tries when you are at the point where the shoes are close and then binding, and backing off a little is a little too much.