Puking brake fluid

[Nyoest]

Had a friend call tonight about his 93 3/4 ton. It's puking brake fluid out of the reservoir. Enough that it has to be topped off every 3-400 miles. He's changed the master cylinder and if anything it's made it worse. He's very competent so I don't think it's an installation issue.



Any thoughts? I've never come across this one before. He also has said that it never has had very firm brakes. (not like I could ever brag about em on any of my trucks)

 

[EDankievitch]

Vac pump failed pressurizing the booster would be my guess.

 

[NIsaacs]

It has air in the system, or old fluid, or both. If the old fluid is contaminated with water, it will boil (at the calipers) and blow out the master cylinder. I would flush all the old out and make sure there is no air.



Nick

 

[Nyoest]

Vac pump failed pressurizing the booster would be my guess.

That was my first thought. I beleive if the vac pump was out it would just be a harder pedal with less braking results.

It has air in the system, or old fluid, or both. If the old fluid is contaminated with water, it will boil (at the calipers) and blow out the master cylinder. I would flush all the old out and make sure there is no air.



Nick

Nick your right. But this fella is about 68 and has taught me quite a bit about cat dozers and tractors. I'm sure he's got the mc on and bled properly. I can't imagine much old fluid left in the system after a mc change but I guess it could happen.



I'm going to take him all your ideas guys, I'm just thinking out loud.



I've been rolling this around in my head since last night and still can't come up with anything new.



Does a first gen have abs? Could it be an abs gone wild?



Thanks for your ideas guys.

 

[NIsaacs]

Yes, the 1st gen. has rear ABS. There is a big 'ol chunk of iron mounted on the left frame rail above the rear axle. It has to be bled first, then the rear brake cylinders.



Not discounting you friend, but sometimes we can't see the forest for the trees:-laf



Nick

 

[Nyoest]

TREES, what trees? All I see is forrest.

 

[JHumphries]

The our ABS doesn't have a pump which is the only way I can think of ABS causing back flow.

It is just a method to cut off pressure to the rear brakes.

 

[JHumphries]

Since I forgot exactly how a brake booster works, I had to look it up.
It would seem that if the vacuum pump were pressurizing the booster, the booster would actually be fighting the driver applying less pressure than input.

 

[cerberusiam]

Vacuum pump doesn't pressurize anything, it uses vacuum to assist apply. There is no pressure there. and it is isolated from the MC by the rod.



If it is actually leaking out of the reservoir, assume thats where the trail of fluid is from, the MC is likely bad. The old ones was probably bad and if he got a reman instead of new this one is likely bad.



The only other way to actually pressurize the system and blow fluid out is a hung front caliper that is cooking the fluid and pressurizing back thru the MC. Should be signs of that somewhere though.

 

[roachcoach]

If he bought a re-man from NAPA or Advanced, it's probably the cap. The rubber seal in the caps on a lot of their re-mans is too small for the cap and doesn't seal on the MC. If you can find a factory cap then the leak will likely stop. Also, if he gets another MC, tell him to try Fisher Autoparts, they sell Wagner MC's. They are a much better quality than NAPA's MC's.