Brake problems

[GRudy]

I have been haveing a problem with the brakes on my '92 W350 for quite awhile now. This truck is my backup vehicle that is used primarily for snow plowing. It is not on the road every day. With that said, here is my problem. I bought the truck back in October last year. Right after that it blew a hole in the left front metal brake line. While the mechanic was bleeding the brakes he noticed that the master cylinder was leaking also so he replaced it with a rebuilt unit. One week later the pedal goes spoungy and the master cylinder was almost empty in both resevours. The mechanic bled the system again and everything was fine for another week or so and then the pedal went spoungy again. This time the master cylinder remained full. Mind you this is all happening now in the middle of winter and everytime I take the truck in it is dripping wet from ice and snow so we figure there must be a leak somewhere that we are missing. Anyhow, now it's dry and we still can't find any leaks but the pedal keeps going spoungy. Through this all we have replaced the master cylinder twice, and the vacuum booster once. Both the front pads and the rear shoes are new. There are no leaks at the wheel cylinders or the disc pistons. What are we missing?

 

[Paul Herioux]

Find the RWAL(antilock) valve. I believe it is bolted to the inside of left frame rail above and just forward of the rear axle on the 92's. On it you should see a shiny(wipe off mud if you have to) barrel shaped part, this is the accumulator. This part should have a rubber plug in the end of it. Remove the plug and insert a thin rod of some kind(a golf tee works great, only real good use for them) till it bottoms out. With your finger on the rod have someone step on the brake and feel for accumulator piston movement, it should NOT move. If it moves it indicates that the dump valve is stuck open, this is normally caused by dirt in the system. If bleeding the brakes has been fixing this condition a full flush of the system may cure it. 90% of the time I end up replacing the valve on my customers vehicles. A dealer I used to work for stocked two of these valve assemblies at one time. They never stocked anything that they didn't sell at least 4 a month of. Get the picture .

If the valve assembly checks out ok try changing brands of master cylinder. I have seen some really poor quality reman cylinders lately , even from the big namebrand parts houses .

HTH's

Paul

 

[NWinnie]

I agree with Paul. It also may be possible that you simply have air in your lines.

Did your mechanic bench bleed the master cylinder by putting it in a vice, filling it with fluid, purging the MC with a dowel?

Or did he install it first and then bleed it?

It could be that you had air trapped in the MC that would only reveal itself after driving (read bouncing) around a little. Either way, you should be sure that after the install of a new MC the entire system should have new, clean fluid in it.

I agree with many of the members here who say,"If you want it done right, you've got to do it yourself.

 

[BCaldwell]

save yoursl the grief if it's not the valve buy a new master cylinder rebuilts are a pain in the you know what. bill

 

[JStoll]

in a master cyl on the rear brake section (drums) there is a check valve that holds a verry slight bit of pressure to keep the wheel cylenders from sucking air when the fluid cools down & to keep them from leaking they arent made to keep air out because of the cup design (one way seal)

this could be part of the problem you may have gotten a bad run of masters

definatly check the acumulator like paul said although I usualy see a bad pedal after bleading & adjusting if the valve/acumulator is bad

it kinda sounds more like you are sucking air to me either through the master or the rear wheel cylenders

JAK

 

[GRudy]

Thanks Guys. This gives us some new places to look. Oo.