brake question (pedal to the floor)

[exmonkeypunk]

I'm pulling my 72' GMC C/2500 into the gas station when my brake pedal goes to the floor and my brake light pops on. I swerved to miss the parked car at the other side of the pump and wiped out a few cones laying around before i was able to coax the 3/4 ton beast to a stop. I looked under at all the wheels and around the lines and saw nothing. I popped the hood to check the fluid level and there was a bunch of brake fluid that spewed from under the master cap. The levels were still full. I dried off the master top and put it back on. Still had no pressure. I have a front disc rear drum truck. Does this sound like a bad proportioning valve? I replaced my right rear wheel cylinder a few months ago but haven't checked the left, but haven't seen any fluid around the drum.

[cparman]

The brakes had been working fine until then? If the pedal went all the way to the floor, and you see no leak my guess would be the master cylinder?????

[exmonkeypunk]

Originally Posted by cparman

The brakes had been working fine until then? If the pedal went all the way to the floor, and you see no leak my guess would be the master cylinder?????

That was my first thought after seeing all the fluid coming out of the top of the master. I've never run into this problem before though. Although, it's only like 15 bucks for a master. Shouldn't i be losing fluid though? i drove the truck around town earlier today and after a few stops, i lost the brakes again and the light came back on. But yeah, before this happened the other day, the brakes were working fine.

[mbgmike]

Originally Posted by exmonkeypunk

That was my first thought after seeing all the fluid coming out of the top of the master. I've never run into this problem before though. Although, it's only like 15 bucks for a master. Shouldn't i be losing fluid though? i drove the truck around town earlier today and after a few stops, i lost the brakes again and the light came back on. But yeah, before this happened the other day, the brakes were working fine.

You can check the MC by blocking off your ports and see if it still works. There should be a firm pedal. If not replace it. More than likely that is the problem Simple fix

[tcb-1]

hummm. wonder if one of the rear wheel cylinders blew a seal? however you said that you checked that.

I'm with everyone else, sounds like a mc.

[sleepertruck72]

I would check the other wheel cylinder as well. I had leaking wheel cylinders in my C20 but they were just soaking the brake linings so no brake fluid loss was evident. I however was losing some fluid in the rear chamber of the m/c, and the brake light would come on. When you replaced the wheel cylinder, did you bleed your lines afterwards ? I agree with mbgmike, try that out.
I think if your proportioning valve was bad, your rear brakes would lock up during your hard braking.

[Bus Ted Knuckle]

bad master cylinder.

[exmonkeypunk]

I'm glad so many agree with the master cylinder probably being the cause of this issue. I've never replaced one before, but by looking at it, as long as all the old lines come off smooth, then it looks like a piece of cake. What bleeding procedure do i need to run through after replacing the master? There's a bleeder on the proportioning valve isn't there? Should i flush all the lines when i replace the master?

[exmonkeypunk]

Originally Posted by sleepertruck72

I would check the other wheel cylinder as well. I had leaking wheel cylinders in my C20 but they were just soaking the brake linings so no brake fluid loss was evident. I however was losing some fluid in the rear chamber of the m/c, and the brake light would come on. When you replaced the wheel cylinder, did you bleed your lines afterwards ? I agree with mbgmike, try that out.
I think if your proportioning valve was bad, your rear brakes would lock up during your hard braking.

I was lucky when the right rear wheel cylinder went out. It was right when i applied the brakes to put it in gear to leave the house. There was almost no mess to hose down. I wanted to replace the left side at that time, but didn't because it looked fine. I bled both rears at that time. A few weeks after that i was replacing the front pads and i tore the right front brake hose. I replaced that and bled both front calipers.

[mbgmike]

Originally Posted by exmonkeypunk

I'm glad so many agree with the master cylinder probably being the cause of this issue. I've never replaced one before, but by looking at it, as long as all the old lines come off smooth, then it looks like a piece of cake. What bleeding procedure do i need to run through after replacing the master? There's a bleeder on the proportioning valve isn't there? Should i flush all the lines when i replace the master?

I don't know how old the fluid is but it never hurts to replace the fluid.. Most times people don't when they should. Bench bleed the MC before putting it om ( I Know there are other ways) install the MC and bleed brakes. Someone else can chime in on the the bleeding procedures

[exmonkeypunk]

Yeah, the fluid is dirty looking, probably rusty. I know the longer the fluid is in the system, the higher acidic it gets. I'm sure i'll end up replacing a few more lines before this is done.

[Daves72'GMC]

Check on the back side of the MC to see if the MC leaked into the booster.It might have destroyed the booster .Check all 4 wheels to see if there is any leaks (with wheels off). Bench bleed new MC ,put it on the truck ,bleed all 4 wheels, You should be good to go.

[exmonkeypunk]

Okay. I've been pretty busy lately. Lost my job and all that. But i finally got around to replacing my master cylinder. I now have pedal pressure, but am hearing a loud hissing whenever the truck is running. I can get it to stop by pulling the vacuum line out of the booster and plugging it with my thumb. Does this sound like my booster is junk? The truck has great pedal pressure but stops kind of lame. I won't be able to buy a booster until next year sometime, so if there's a way i might be able to rebuild it or whatever. let me know.

[Longhorn Man]

Your booster is junk
Did you have to fight the pushrod in there at all? I've done that one myself on a friend's truck.
you can remove the booster and bolt the master cylinder directly to the firewall. You'll need a new pedal to master cylinder pushrod.

[exmonkeypunk]

Originally Posted by Longhorn Man

Your booster is junk
Did you have to fight the pushrod in there at all? I've done that one myself on a friend's truck.
you can remove the booster and bolt the master cylinder directly to the firewall. You'll need a new pedal to master cylinder pushrod.

where can i find a new pedal to master cylinder pushrod at? didn't know i could do that. do i just block off the hose then? The new master bolted on with no issues.

[exmonkeypunk]

Is this pedal to master pushrod something i need to make? i haven't found anything so far online.

[CCCC]

Sometimes getting the correct length pushrod can be a pain, and I have found the length to be critical to correct operation. You probably could make one with rounded ends just fine, but what exact length.? Without a model to work from it could be tough. A good brake shop (find an older guy) should be able to advise easliy - and maybe reach into a box of used ones and hand you the correct one. I save every one I find.

Actually, IMO you may not be ahead by removing the booster and bolting the MC directly to the firewall. In my slipping memory, seems like braking action through a non-functioning booster is about the same as non-power brakes. Maybe not?

If so, save yourself the pushrod problem and drive as is (with caution) until you get a working booster. There is an outfit in the nearest big city here that rebuilds them in an hour or two for about $100. And, they appear on this site every so often in good used condition for much less Make sure you get the correct one for a 3/4 ton - different from the 1/2 ton and better action - and the booster for the 1 ton is different yet (even better IMO) and might fit your 3/4.

The advice from Daves72 and others here is very good.

Best wishes,
CCCC

[exmonkeypunk]

Originally Posted by CCCC

Sometimes getting the correct length pushrod can be a pain, and I have found the length to be critical to correct operation. You probably could make one with rounded ends just fine, but what exact length.? Without a model to work from it could be tough. A good brake shop (find an older guy) should be able to advise easliy - and maybe reach into a box of used ones and hand you the correct one. I save every one I find.

Actually, IMO you may not be ahead by removing the booster and bolting the MC directly to the firewall. In my slipping memory, seems like braking action through a non-functioning booster is about the same as non-power brakes. Maybe not?

If so, save yourself the pushrod problem and drive as is (with caution) until you get a working booster. There is an outfit in the nearest big city here that rebuilds them in an hour or two for about $100. And, they appear on this site every so often in good used condition for much less Make sure you get the correct one for a 3/4 ton - different from the 1/2 ton and better action - and the booster for the 1 ton is different yet (even better IMO) and might fit your 3/4.

The advice from Daves72 and others here is very good.

Best wishes,
CCCC

Thanks for the replies on this everyone. I was able to put the truck in storage for the winter in a neighbors garage. I have all winter to get money together for the booster. The local parts store can get reman ones for around 100 bucks, so i'll just go that route. Hopefully, this spring i'll put it on and maybe have great brakes again.

[mbgmike]

Originally Posted by exmonkeypunk

Thanks for the replies on this everyone. I was able to put the truck in storage for the winter in a neighbors garage. I have all winter to get money together for the booster. The local parts store can get reman ones for around 100 bucks, so i'll just go that route. Hopefully, this spring i'll put it on and maybe have great brakes again.

Sounds like a plan. Just for info we sell just about any thing as far as brakes.New and reman. We sell a variety of brake rods, clevis.etc to hook up anything. If not we make them. Any machine shop can make you one. Its not really an exciting piece or a challenge to them but a nice one will do it for you. If you need dimensions on that kind of stuff just ask
JFI I am old