Question for brake gurus...

[Greenlee]

I suddenly have brake problems on my 76' Chevy after pulling a tandem axle trailer and tractor weighing probably 4,000 lbs. Coming up to a red light and getting on the brakes pretty hard, the pedal went almost to the floor and the brake light came on for a second and went off. Before this incident, the brakes were fine. After unloading the trailer I noticed the pedal was still going almost to the floor and the red brake light came on again and stayed lit. I check the brake fluid and the big reservoir was a little low. I filled it with fluid, but the light is still on and the brakes are still working poorly with the pedal going almost to the floor.

The truck is not pulling to either side, and I have checked all four wheels and don't see any obvious signs of leakage. My first thought was that maybe the master cylinder has gone bad, but I am not sure how to test it. Does anyone have any ideas?

Thanks

[knightlife]

check your pads for wear? any noise? Im bettin you are right about the master cylinder... but I are no jee-nee-oos

[89 crew]

Check again underneath for leaks. Look at the rear wheels for fluid running down. If there isn't any leaks replace master cylinder. The only other possibility would be a swelling rubber line. If it is swelling it will only do it when someone is pushing the peddle. Which brakes currently work? front or back?

[Greenlee]

Originally Posted by knightlife

check your pads for wear? any noise? Im bettin you are right about the master cylinder... but I are no jee-nee-oos

No strange noises. I went from good brakes with good pedal feel, to bad brakes and pedal to the floor in a matter of minutes. I haven't checked pads or shoes yet. I have a different rear axle with lower gears that I was about to swap in, so I will be going through the rear brakes anyway. The brakes were working so good a few days ago, I was thinking about re-using them on the new axle.

[Greenlee]

Originally Posted by 89 crew

Check again underneath for leaks. Look at the rear wheels for fluid running down. If there isn't any leaks replace master cylinder. The only other possibility would be a swelling rubber line. If it is swelling it will only do it when someone is pushing the peddle. Which brakes currently work? front or back?

No fluid running down, but I will investigate this further and check the lines for swelling. Thanks for the tip. I think the front brakes are working better at the moment.

[jermasaurus]

It can be difficult to spot a leak sometimes. A common spot is on the left framerail where it would go next to the auxilary fuel tank. Mine was where the line went through a bracket on the frame.

Could also be the master cylinder. But if your brake light came on too, I would think the rear isnt holding pressure and the pin on the proportioning valve came out.

[89 crew]

A leak that is good enough to cause the proportioning valve to shift shouldn't be that hard to see. just get in a position where you can see the majority on the bottom of the truck and have someone pump the peddle. if there is a leak you should see it spray. Don't forget to check up around the proportioning valve, and over by the starter for leaks. The last 2 blown brake lines I fixed were in that area.

[James McClure]

Greenly: When you pulled the trailer you forced the brake system to work harder. Greater load = higher pressures = weak spot going to give out. OK, you have determined the front brakes are holding and the rears aren't SO, the leak is going to be in the rear system. The #1 probable failure is the lines (steel) going to the back. Seeing as the light is on, the proportioning valve has shifted shutting off fluid to the back. Rear chamber on master cyl empty or nearly so. Line for rear brakes runs from the proportioning valve on the back of #1 cross member just behind the steering gear box, across to the right side and down the frame rail to a height sensing valve OR, a block where the rear brake hose attaches to feed rear brakes. Check these lines carefully, you'll find the wet spot. General rule of thumb is, if you have to change the steel lines, replace the hoses too.
If you find the lines are OK and there is NO sign of the hose blistered, pull the rear drums and look at the wheel cyls. Wheel cyls usually don't fail in a way that drops the pedal all at once, but leak out over time. That doesen't mean they haven't failed though. If EVERYTHING looks good, THEN go to the master cyl. You can check it with a pressure guage if you can rig one up and be able to bleed it so pressure readings are accurite. You'll have to use a high pressure guage, I would guess something over 500 PSI might do it.
I don't think your going to have to go that far though. I replaced lines on mine twice before I grew a brain and replaced them with stainless steel ones from Classic tube. If you want more info on classic tube, go to "classictube.com". or drop me an e-mail at "jmcclur2@nycap.rr.com" and I'll fill ya in. Good luck, hope this helps jim

[streetstar]

change the master cylinder and re-bleed the complete system (at all 4 wheels)

relace anything that looks bad at this time --pads, lines, etc. if you can

but i would bet a 3 dollar bill its the MC

[TexasJeff]

In my opinion you blew the seal in the master cylinder. Either rebuild the master cylinder or buy a new or rebuilt one, then bleed the entire system and I think you will find you fixed the problem.

[James McClure]

Greenly, What happened? Did you find the problem?? jim

[bbchevy]

Pull the drums and see if a spring fell off of the shoes.... if the brake shoes were out of adjustment, there is always a chance that one of the springs has fallen off. that would cause instant pedal loss. just an idea. its worth checking that way you can check the wheel cyl too

-Scott

[GMBowtie79]

sounds like you might have blown a wheel cylinder out pull the drums off

[89 crew]

Originally Posted by GMBowtie79

sounds like you might have blown a wheel cylinder out pull the drums off

If he blew a wheel cylinder, it would be leaking brake fluid on the inside of the tires. He said it is dry there.

[N2TRUX]

Master cylinder is blown "internally" so it won't be leaking fluid....