Master cylinder pressurized?

[jerseyjeep2003]

The vent line to the master cylinder on my 71 deuce is pressurizing instead of venting. It has caused my 6 month old MC to constantly leak. I have a kit to rebuild the MC and a kit for the air pack. Am I heading in the right direction with this? What is supposed to be at the vent end of the line by the air cleaner box. Mine has what appears to be a brass plug. Thanx guys for your knowledge.

 

[Warthog]

The M/C should not be preasurized.

I would think that there is something wrong in the vent tubing (someone installed inccorrectly).

Rebuilding the M/C and Airpack are always a good idea. Nut the issue will come back.

 

[Floridianson]

Do believe the MWO called for it to be removed from the line the connects to fuel vent line and road draft tube. You can seperate the line and just run it up the fire wall with a valve like are on the axles. You could be picking up a little pressue maybe from the tank if not venting correctley or the road draft tube has a lot of blowby maybe? Also if your shure you will not get deep in the water you can just put the one way vent right on top of the master cyl. cap.

 

[jerseyjeep2003]

I'm thinking about adding a remote reservoir in the cab, I saw a thread on here about it once. The pressure seems directly related to the pressure in the air tanks. I intentionally left the fitting on top of the MC loose and I could hear it hiss even after the motor was shut down. That is why I wondered if there could be backpressure leaking at the air pack somehow.

 

[doghead]

That is why I wondered if there could be backpressure leaking at the air pack somehow.

Yes, rebuild the Airpack.

 

[Speedwoble]

That brass "plug" may be a sintered brass vent. The center of it is a sintered filter element to keep dirt out.

 

[jerseyjeep2003]

Yeah...rebuild of the airpack is on my to do list tomorrow, and the MC too, already have them out and sitting on my bench. Hmmmm, the sintered vent almost makes sense if it is clogged would have to releive any pressure somewhere...like into the MC. I am just supposing, because it really doesn't make any sense that there is a build up of pressure in the MC.

 

[Vintage iron]

your have the right idea. It sounds like the airpack is leaking into the hydraulic (brake) side. Take plenty of pictures during the rebuild. We would like to see them. good luck!

 

[rlwm211]

My guess is that the vent is getting pressurized upstream from the vent and not from the air pack. Other than a catastrophic failure in the Airpack I cannot envision air getting into the master cylinder from there. It would displace all the brake fluid which would be quite noticeable as it would be all over the place. On top of that, there would be no brakes.

I suggest you follow the vent line up into the engine compartment and see if it is connected to something it should not be.

Rebuilding the brake components on these is always a wise idea as these trucks typically have been sitting with little or no use for a long time. It does not take long for things to start deteriorating and becoming unserviceable.

When I got my truck I started piecemeal and ended up replacing or rebuilding everythng in the brake system. I did the PCMS that needed to be done at the same time so it was worth the effort as I have had no issues since.

 

[doghead]

Simple test, take the vent line off the MC and run the truck, where is the air coming from(mc vent or the line that is no longer connected to the mc)?

I've had 2 trucks that did the same thing, both were airpacks bad.

 

[jerseyjeep2003]

Yes, rebuild the Airpack.

Dog, you are one of the members whose opinion I value. So, a bad/leaky airpack could backfeed pressure into the MC? The rest of my braking system appears to be in good shape. I even debated rebuilding the MC, because I had it out and apart about 2 months ago and it was clean and smooth as a baby's behind inside. But, since it is out for the airpack rebuild I may as well freshen it up at the same time.

Oh....almost forgot when I was diagnosing it the other day, I had the cap off the MC, and slowly pumped the pedal. Typical to any other MC the fluid moved in and out of the port hole in bottom of the cylinder. I was OK with that and turned away to do something else....... well, I heard this splashing sound. I turned back to see what it was, and a stream was spraying out of the top of the MC and actually reaching the ceiling of the cab. What the heck!! I wasn't even near it. Doesn't make sense.

 

[rlwm211]

Airpack...DH is right

 

[jerseyjeep2003]

Simple test, take the vent line off the MC and run the truck, where is the air coming from(mc vent or the line that is no longer connected to the mc)?

I've had 2 trucks that did the same thing, both were airpacks bad.

I'll have to double check that one, but, I think it is coming from the vent line.

Can't hurt to rebuild since I have the kits.

 

[jerseyjeep2003]

Airpack...DH is right

I had my suspicions, but love getting expert knowledge. I can't wait until I get up tomorrow (working the vampire shift) so I can tear into the rebuild.

 

[Floridianson]

Are you down on brake fluid to. Would seem that you would also lose some when you do apply the brakes. If like you said the pressure is there with the cap off then yes it would seem the airpack is leaking back. Easy check.

 

[jerseyjeep2003]

AAARRRGGHH, Master cylinder rebuilt, Airpack & slave cylinder rebuilt, added remote reservoir, and pressure bled system. Nothing, nada, nunya, pedal still goes to the floor. I had massive amounts of air at the wheel cylinders, but finally got straight fluid under pressure. But still no pedal. OK, thought about bench bleeding the master, but, it should have been OK since I bled it at the air pack. I then discovered a missing copper washer at the output of the master cylinder. Thought that was the problem, NOPE!! I am at wits end here. Should I just continue to bleed the system, or put on a different master cylinder I have ( it was oozing fluid even though it was new)? Any thoughts from those smarter than I?