Brake expert help needed!!!!

[bcj67chevy]

I have a 1972 chevy truck w/ power brakes, disc up front and drums in the rear. Only had the truck for a couple weeks, brakes were terrible.

First of all when the truck is off the pedal is rock hard form the get go and only goes down a couple inches.

Changed the rear brakes: shoes, wheel cylinders, all hardware, and brake lines from rear hose above the diff. (front pads and rotors looked good visualy)

Tried to bleed the rear brakes and can't get it to work, try to pump up the brakes but the pedal only goes down about two inches before it gets rock hard(from the very first pump), then when i hold it down while the bleeder is broke loose the pedal doesn't go down and barely any fluid comes out.
If i do the same thing with the front i get the same hard pedal but then the pedal does go down. Could only do the one front because the bleeder broke on other side.

I even tried to bleed it directly at the master cylinder, and same thing.

Is something messed up with the front that it isn't letting the rear work??
What would cause the rear brakes to not bleed??

Any advise would be appreciated.
Thanks,
Bryan

[truckdude239]

the portaning valve is shut off fuild tot eh rear the valve needs to be reset

[kcjones]

I just went through this on my truck -- combination valve has "popped", meaning the valve has moved into a position to only allow pressure to one side (in your case, the front). The rear brakes are being blocked off, and no pressure is going to them. No amount of standard bleeding is going to help. You must pop that valve back into place.

There is a standard procedure, and about 15 variations on it depending on a number of factors. The standard procedure is this:

Make sure the brake warning switch is on - this will tell you that in fact the combo valve is off center and blocking the rear brakes. Hook a test light up, connect the ground to the switch contact. If the light lights, your valve needs to be recentered. If it doesn't light, there may be something else going on.

To recenter the valve, you'll need to apply the brake pedal while bleeders are tight in the rear, and loose in the front. To do this, go through a normal bleeding of the front brakes, but be prepared to cycle quite a bit of fluid through the front lines to get that valve recentered. At some point, the valve should recenter, the light should go out, and you should be able to get normal "squirting" from your rear bleeders while bleeding. Once done, follow your normal bleeding procedures to bleed the system.

Things to note!!:
-Some combo valves have a button in the front that has to be pressed/held in while bleeding.
-If there is some loss of pressure in the rear brakes (tons of air, lines not tight, bleeders not tight) while you're trying to "unstick" your combo valve, the valve will just continue to uncenter.
-There are a number of other threads on this forum that are VERY helpful if the standard stuff doesn't work. I've read them all.

....AND MOST IMPORTANT!! I had to bench bleed my master cylinder even though I thought there was no way I should have had to do that. Once I did, I was able to reset the combo valve, bleed the brakes normally, and all is well. Took me a while to see the light ;-)

[cdowns]

factory service manual has loads of info is cheap and even tells you the proper way to bleed the brakes comes with pictures too// available from most board vendors at cheap prices a far better investment than any magazine you waste money on

[bcj67chevy]

If it was the prop valve then why can't i bleed the rear line at the master cylinder??

Thanks for the advice.

[68gmsee]

You may be looking at defective vacuum booster or something wrong with you M.C. If it's a new one, it may still be defective. I've had to take brand new ones back.

[bcj67chevy]

I just changed the mc and got the same result as with the old one.
if the booster was bad i should still be able to bleed the brakes right??

If a hose was collapsed in the front(on the side that i couldn't bleed) could that di it??

[jholley]

A little off topic but rebuilt calipers are like $15 at O Reilly's. The core is more expensive than the caliper itself. Maybe one of yours is locked up or clogged up?

[68gmsee]

Originally Posted by jholley

A little off topic but rebuilt calipers are like $15 at O Reilly's. The core is more expensive than the caliper itself. Maybe one of yours is locked up or clogged up?

(He says he can't even bleed at the master cylinder.)

I'd say that definitely the booster could be a problem. Collapsed hose would still let you bleed the other wheels and would not cause the rock hard feel after 2-3 inches. Usually they cause wheel lockup.

Be sure and have the engine running when checking the booster. It won't work too good otherwise.

[jholley]

I'm just sayin, busted bleeder, drill, tap, new bleeder... or new caliper. Strange the new master cylinder has the same problem as the old one though as far as "bench bleeding"

[flashed]

I am having the same problem with my 72 1/2 ton truck, pedal hard as a rock. I drove to work this morning and all was fine until time to go home .I recently changed from manual disc to power disc by installing a new master cylinder,booster ,and prop. valve assembled by Frank (pickmup) .I also replaced both wheel cylinders and brake shoes and the rear hard lines and hose due to leaking. Everything on front looked good but I put new pads on just to be safe. Everything was bled good and working properly . I will try several things tommorrow to see what happens.

[bcj67chevy]

Thanks guys!!!
Only thing that doesn't make sense to me is that even if the booster is bad that shouldn't affect me being able to bleed the brakes since the vehicle doesn't have to be running to do so.

This problem just doesn't make sense at all..

Thanks guys!!

[steelhorse]

The hard pedal should be the booster.Not being able to bleed both sides of the master is either a bad master or possible the booster isnt letting you get a full pedal stroke.Is this a new setup or an existing one?

[bcj67chevy]

This is a existing setup and i've only had the truck for a couple weeks to boot.
I just don't understand how the booster can affect anything when the vehicle is off.

I did check the booster with a hand vaccum pump and it held 20psi for 30 min. before i unhooked it. So the diapham is good in the booster.

I would wonder if it was a linkage if the pedal wouldn't go down at all, but it acts correctly when the front is bleed.

Thanks.

[OhOneWS6]

I am having similar issues with mine. In fact for 20 years my truck had the issue you described. My situation is a little different now since I have done a 4 wheel disc conversion. My brakes however are still lacking.

One thing that made a big difference for me was adjusting the linkage to the pedal. Not the booster push rod but the actual linkage to the petal. My pedal was bottoming out before the brakes were fully engaged. This made a huge difference in bleeding them as well as how they feel. Just something you may want to try.

With you being able to bleed the front and no pressure in the rear it still sounds like a proportioning valve issue to me.

[bcj67chevy]

I will have to check the linkage, the pedal travels quite far when i bleed the fronts, like 6+ inches.

We'll see what happens.

Anymore ideas to try also??

This may be a stupid question, but if the front brakes were capped off at the master cylinder the rear brakes couldn't be bleed because the front fluid would get so compressed in the front(at the mc) that the pedal couldn't go down far enough, right.

I would say it was the prop valve, but if it was than it would block fluid so that i couldn't bleed the front, right.

This old of a truck doesn't have crossed brake lines does it? Front line on mc does just front brakes right?

Thanks guys!!

[mrein3]

Originally Posted by bcj67chevy

snip


Anymore ideas to try also??

snip

IF your prop. valve is allowing fluid back there and you aren't getting anything out of the bleeder screw, take your bleeder screws out and make sure the passage in there is clear.

Up here in the rust belt I've more than once had to take them out and clear them with a little drill bit that I roll around in my fingers.

Just last week while replacing the rear brake hose on my truck I snapped off a bleeder on the LHS of my truck. Last factory original piece on my truck with a bleeder. In my book a broken bleeder means either a new caliper or wheel cylinder. NEVER skimp on brake parts.

And you folks out of the rust belt have no idea what we go through to perform a task like this.

[cloudbrk]

Originally Posted by cdowns

factory service manual has loads of info is cheap and even tells you the proper way to bleed the brakes comes with pictures too// available from most board vendors at cheap prices a far better investment than any magazine you waste money on

Who on here has these? I need to get one and would love to have someone scan the brake bleeding procedures and pictures section so that I can get that done before the book arrives. Anyone?

[bcj67chevy]

Originally Posted by mrein3

IF your prop. valve is allowing fluid back there and you aren't getting anything out of the bleeder screw, take your bleeder screws out and make sure the passage in there is clear.

But i can't even bleed the rear by cracking the line loose at the master cylinder. The pedal won't go down to let fluid out.
But if i do the front at the master cylinder the pedal goes right down.

[red71cheyenne]

In the rear there is a "T" where the lines split off to each side. When I first got my truck, I had to replace the T because it was doing the exact same thing. I'd pull the incoming line off of it to see if you get fluid out prior to it and go from there.

[smn69]

I had a problem bleeding my brakes and could not figure out what the deal was. It was a new MC, did the bench bleed and everything, but couldn't bleed the back brakes. I checked everything and finally noticed that the piston in the MC was binding and not returning all the way back after pressing the brake. Returned the new MC for another new MC and the problem was solved. Good luck with your problem. (Call me sick, but sometimes the fun is in solving these perplexing problems.)

[mrein3]

Originally Posted by bcj67chevy

But i can't even bleed the rear by cracking the line loose at the master cylinder. The pedal won't go down to let fluid out.
But if i do the front at the master cylinder the pedal goes right down.

Then, as others have pointed out, your prop. valve is stuck in the fail safe mode only allowing the fronts to work.

The other day when my rear rubber line blew I still had brakes and my "brake" light in the dash lit up for the first time for me in 16 or 17 years of ownership. You might want to test that light too cause the bulb may be burnt out.

[bcj67chevy]

Originally Posted by mrein3

Then, as others have pointed out, your prop. valve is stuck in the fail safe mode only allowing the fronts to work.

I must not be understanding something, if I try to bleed the rear brakes at the fitting that connects directly to the mc, before the fluid can go thru the prop valve, then how can the prop valve affect the fluid that is is coming directly out of the mc?

I'm not trying to be difficult i just don't understand!!

Thanks guys for being patient!

[LOW DOUGH]

Interested to know what you find.

[mrein3]

Originally Posted by bcj67chevy

I must not be understanding something, if I try to bleed the rear brakes at the fitting that connects directly to the mc, before the fluid can go thru the prop valve, then how can the prop valve affect the fluid that is is coming directly out of the mc?

I'm not trying to be difficult i just don't understand!!

Thanks guys for being patient!

I've never had to do this procedure on my truck but I did have to do it on my Chevelle when I put my dearly departed 71 Monte Carlo front power disks on it.

In the service manual there is a diagram of the tool you need to make. It bolts to the proportioning valve and holds the button on the front of the proportioning valve for you. I'm guessing here but if that valve is stuck backward because of a loss of brake pressure to the back brakes, holding the button allows the valve to re-center. I agree that it doesn't make that you can't get any brake fluid to squirt out upstream of the proportioning valve but before I do anything else I'd follow the procedure in the manual to see if things straighten out.

Right now I'm grateful for that little valve in there. When my rear brake hose blew the other day I still could limp home and know my fronts were still operating. Before that little valve came along, when you lost one component in the system you lost the whole system.