mysterious brake failure

[GRudy]

Help, need insight. About 2 months ago a brake line blew a leak on my '92 W-350 so I had it replaced. While the mechanic was bleeding the brakes he noticed a leak at the master cylinder so he replaced it also. Then a couple weeks later the pedal all of a sudden goes to the floor. Pumping the pedal gave enough brakes to baby the truck home and back to the mechanic. Thinking another line blew we searched everywhere for another leak but could not find one. Bleeding the brakes over again gave a good solid pedal again and everything worked great for another two or three weeks so we figured he just didn't bleed out the new master cylinder correctly. Then a week later the pedal goes to the floor again. Still no sign of any leaks anywhere, lines or wheel cylinders, everything was dry. So where is the brake fluid going to? This time we replaced the vacuum booster thinking the the fluid was being sucked into the engine and burned. Low and behold a week later the pedal goes to the floor again. I should mention that each time the brake fluid was equally low in both master cylinder resevours. Again, no leaks can be found anywhere. Where can the brake fluid be going? I'm at a loss.

 

[Philip]

If you have no visable leaks showing. The only place left is the seals on the master cylinder are leaking into the booster. Then the vacuum pump is dumping it out. The pump is low enough that if it is coming out there. It might not hit any part of the suspension to show a leak.



Are you getting air out of the lines when you bleed them out?

 

[formula]

George 00,

Your replacement master cylinder could have a bad rear seal. Is it new or a rebuild? Does it have a warranty? I had a similar leak in my old master cylinder. Look under the back of the master cylinder where it attaches to the booster. Is it wet with brake fluid? Is there brake fluid on the frame rail under the booster?



It is not possible for the engine to suck brake fluid through the booster and burn it. The vacuum pump on the engine vents into the crankcase, not the intake. Also, you'd need to have a couple of quarts of brake fluid inside the booster to reach the level of the vacuum port.



Sean

 

[GRudy]

Thats what we figured and why we replaced both the master cylinder and the booster. This thing has me stumped. Is there anywhere that a brake line runs inside a frame rail or any other place where a leak could go undetected?

 

[Philip]

The only place the line is hid is behind the fuel tank.



I have seen brake fluid go threw a booster before.



Is the antilock valve still hooked up? It is located on the left side frame rail in fron to the rear axle.

 

[GRudy]

The last time I took the truck into the shop I stoped at the local hydrospray and cleaned the truck underneath. We put it up on his lift and let it sit there all night in his heated garage to drain and dry. The next day while a guy was in the truck literally standing on the brake pedal he went around underneath with a light looking for a leak and could find nothing.

To the best of my knowledge, both the replacement master cylinder and the booster unit were remanufactured parts. So far with the three times the brakes have failed we have lost about a quart of fluid from the master cylinder. (not counting what has been used to bleed the brakes) As for the ABS valve, good question, I'll look. You gotta understand it's been very cold here the past month and evertime the mechanic has done the searching in his heated garage. It's time I go myself and search for the leak. He has to be overlooking something. Either that or find another mechanic!

 

[Philip]

Loosing tht much fluid you should see something. The only other thoughts is the pads wearing that fast. Do you do a lot of heavy towing?

 

[GRudy]

No, not during this episode. I use this truck now strickly as a plow truck. so yes it's hauling loads and the brakes get used allot, but we have only put about 500 miles on the odometer since this started.

And I forgot, when it was in the first time for the first brake line leak and the master cylinder he also put new linings on the rear drums and pads on the front. the entire system is now new except for the metal brake lines. Thinking about this now, I wonder if he ever pulled wheels after the first trip in. He is probably thinking that if a wheel cylinder was leaking he should be able to see it outside the backing plate or on the tires. Like I said, I think it's time I start diesecting this thing and look at it from a new perspective. the mechanic is aparently overlooking something.

 

[JLEONARD]

To the best of my knowledge, both the replacement master cylinder and the booster unit were remanufactured parts

I would suspect a faulty rebuilt unit if you can't find any external leaks. Been there. Will never buy rebuilt again.

Jay