Another Brake Question

[72Much]

Ok guys. I need some help here. I'm lost as last year's Easter egg. I thought brakes were fairly straight forward and easy to deal with but I stand corrected.

My '72 C-10 is a half ton two wheel drive truck. Original 307/350. Years ago the original rear took an early exit. A rear out a car was put in and made to work for probably 20 years (don't ask me why my dad did that). I located another original 5 lug rear from a '71 truck and replaced it. When I replaced the rear I put in new wheel cylinders on both sides. I then found a donor truck's original power brake brackets, etc. and mounted those. Compliments of O'reilly auto parts I have a new brake booster and master cylinder.

Ok. Here goes. First off I bench bled the new master cylinder. NO AIR in this thing whatsoever. After hooking everything up and attempting to bleed the brakes I'm not getting any pressure to the rear of the truck. I know to work the farthest from the master cylinder forward. I thought the master cylinder may have been bad. So, I'm on my third master cylinder. I worked my way from the rear of the truck to the front breaking the lines at each junction (my dad was in the truck applying pedal each time). I finally got aggravated at not having any pressure and went directly to the master cylinder. While the brake was applied I broke the line loose as it initially exited the master cylinder body for the front brakes. Fluid came out as expected and the pedal began to fall under my dad's foot as it should. I closed it back while the brake was continuously applied to prevent any air from entering the master cylinder. Now on to the pesky back line and back brakes. The same scenario was done and only a few drops would come out with the brake applied and the line completely loosened. How can this be?? I now know that I'm not getting ANY brake fluid to come out of the back of the master cylinder with the brake applied. This is as far as I've gotten. Can someone please advise what I'm doing wrong??

Thanks a ton guys. I'm past ready to cruise my old truck.

[gonzo6869]

Have you check your proportioning valve, bet that could be your problem.

[72Much]

Gonzo, I took another prop valve from a donor truck I just bought. I tried that also. The thing is, when I loosen the fittings directly out of the master cylinder, before the lines even get to the prop valve, I still have no fluid coming out to bleed. Why can I get fluid to come from the front line but hardly a drop from the back line??

[72lb4x4]

In the block under or to the side of the MC where the lines go, there's a part at the front that should be kept from moving with a sort of L bracket when bleeding.

Is your brake light in the dash on all the time?

You may be able to get some fluid in the front and rear and jump on the pedal and get the piston in the block to go back where it belongs.

A less moronic, but somewhat similar explanation can be found in the manual, or on the web. I'm a bit cooked from working under my junk car and can't explain it well.

[72Much]

72lb4x4,
I have replaced my prop valve from a donor truck. My old prop valve did have the brake light on in the dash. This donor part now has the light out. Please understand, I'm trying to get fluid to come directly out of my master cylinder BEFORE it even gets to the prop valve. I'm not getting a drop from the rear of the master cylinder. The front is pushing fluid out nicely as it should and the brake pedal falls to the floor under pressure from my dad's foot. The exact same process was tried on the back line and hardly a drop comes from the master cylinder.

Again, the MC was bench bled and mounted as per the instructions. This is my THIRD MC on this truck from O'reilly auto parts. The power brake booster is new also.

Surely somebody can help me figure this out. Thanks again guys for all the input.

[72Much]

ttt........somebody help please!!

[dmack91]

I hate to say the obvious, but if you get fluid from the front MC port, but not the rear, it sounds like a bad MC again .

Maybe get one from NAPA in case Oreilly's had a bad run.

Or, is there any chance the line from the rear port on the MC to the prop. valve is plugged up and not allowing any fluid through? Sometimes on trucks this old, sediment can build up an plug the lines. You may want to blow all the lines out with compressed air before trying to bleed them.

[mbgmike]

did you get it fixed?

[Tim R]

check to see is the rod in the new brake booster is the correct length . There are a couple of different lengths .The rod that pushes the master cyl coming out of the booster may be to short and not pushing the master all the way in

[72Much]

Tim, that is my next thing to do. If that doesn't work, I'll take my dad's advice. I bought a short steel line with the correct fittings. I'm going to hook the front MC line that is working correctly to the rear with this "jumper" line and bleed the rears. Then hook up everything again and see if it was just a pressure issue. If all this fails then it's off to the store for my 4th master cylinder. That's all it can be.

Anyone think of anything else??

[Autoblueprint]

I'm trying to get fluid to come directly out of my master cylinder BEFORE it even gets to the prop valve. I'm not getting a drop from the rear of the master cylinder.....Again, the MC was bench bled and mounted as per the instructions.

If I understand you correctly : you have fluid release from rear port when bench bleeding but when you install master you have no fluid out of rear port when breaking the line at the master cylinder?

If that's the case then it is probably a brake rod adjustment issue.

If the rear port squirts fluid when bench bleeding but does not squirt when installed on the booster it is a rod adjustment problem.

You should only have about a quarter unch free play in the brake pedal. If you have more, then adjust your brake pedal rod.

[dutchmanz]

I was thinking the pedal adjustment inside the truck as well If the pushrod coming out of the booster is adjustable, it could be to short too. Make sure you are using a MC for power disc also. The manual disc has a deeper pushrod hole.

[Autoblueprint]

Looked in the Factory Service Manual for my 1971 C-10 RE: bleeding brakes pg. 5-25 and there is a NOTE saying:

the pin in the end of the metering portion of the combination valve must be held in the open position and not allowed to close using special tool J-23709.

If this is not done then no brake fluid will flow to the rear.

If you have held the pin open and still do not get fluid...then the metering valve needs to be replaced...and the valve is part of the combination valve...so the combination valve assembly will have to be replaced.

Hope this helps.

[BIGKEV]

Originally Posted by Autoblueprint

Looked in the Factory Service Manual for my 1971 C-10 RE: bleeding brakes pg. 5-25 and there is a NOTE saying:



If this is not done then no brake fluid will flow to the rear.

If you have held the pin open and still do not get fluid...then the metering valve needs to be replaced...and the valve is part of the combination valve...so the combination valve assembly will have to be replaced.

Hope this helps.

In most case if the combi valve is bad I just gut them. Some aren't serviceable, some are. Also have you tried a simple gravity bleed? Sometimes it's a pain until the fluid trickles through the lines. I like to vacuum bleed, but for some that's not an option.