Air in front brakes when bleeding, no fluid leaks.

capt jake

capt jake

New member
Its a 78 Scout II with the 345 auto trans. Rear drums bleed normally. Front disc brakes blow air from bleed screw and some fluid. There are no fluid leaks from anywhere in the front system that I can see. The pedal starts hard and then softens to the floor until the rears engage and the "brake" light illuminates. Pumping the pedal restores pressure to the front. Cannot find the leak. What puzzles me is that I do not lose fluid in the front reservoir unless I bleed them. Bottom line is I cannot see where I am taking in air. Troubleshooting recommendations and parts list if able. I dont want to just start throwing money at the problem. Thanks.

Jake
 
How's yuma doing? I lived there for about 10 years and loved it. Don't ever remeber seeing a Scout there back then...

scouts have a little button on the porpotioning valve under the master cylinder. This button needs to be pushed or pulled (I forget) when bleeding the front brakes. Hth
 
capt jake said:
its a 78 Scout II with the 345 auto trans. Rear drums bleed normally. Front disc brakes blow air from bleed screw and some fluid. There are no fluid leaks from anywhere in the front system that I can see. The pedal starts hard and then softens to the floor until the rears engage and the "brake" light illuminates. Pumping the pedal restores pressure to the front. Cannot find the leak. What puzzles me is that I do not lose fluid in the front reservoir unless I bleed them. Bottom line is I cannot see where I am taking in air. Troubleshooting recommendations and parts list if able. I dont want to just start throwing money at the problem. Thanks.

Jake
Click to expand...

It is entirely possible that your master cylinder is "by-passing" a very common failure mode and an issue I see quite often when installing a "rebuilt" master cylinder that in actuality is a pos!

Your disc/drum system uses a combination-type proportioning valve, one segment provides the switching for the warning light, the other segment provides a balancing of the front-to-rear braking bias.

To properly bleed this type brake system, the small contact button found on the end of the valve must be partially depressed. That allows the internal spool to recenter itself once the pressures between the two segments of the valve equalize and turn the warning light off. To do this it takes either two persons (if doing the pedal-pump bleeding method), or rig a small c-clamp carefully (or a tiewrap, or whatever) to partially depress the button, do not hold the button fully depressed!

If the button is frozen, then the valve itself is scruud and should be replaced.

So as for diagnosis, bleed the system fully using the procedure. This May have to be done several times over the course of a few hours if you are not using a power-bleeder system. If the brakes still won't come to life, I'd venture the master cylinder is in the crapper and can't build proper pressures in both sides of the system.
 
Yeah, I was wondering what that button did. However, it pulls out about 1/8th of an inch but will not push into the prop valve. It definitely sounds like its hitting something. The button is not frozen, there is something blocking it from moving inward at all. Is there an internal slide valve inside that could be frozen? Anyway, so far it sounds like the prop valve is afu. I'm ready to start there.
 
If you can't move that button either in or out, then the spool valve is frozen and that unit needs to be tossed! This one will pull out a bit but not push in which is what is needed to reset the warning switch. So this valve sample I have here is also failed.

You can screw around with disassembly,...you can prolly get the "proportioning" segment out but these valves cannot be serviced.

If you disassemble it and try to put it back together, no doubt it will leak....and if it has any contamination (which is why it's frozen!) that can definitely inhibit brake operation. Messin' with these valve systems is not worth the risk!

I just went out to disassemble a rotten one so I could show ya what we're talkin' here...but the "button" module broke off right away, the spool valve is frozen, so I would not have been able to totally dismantle anyway.
 
Im kinda in the same boat. My rear resevior keeps on coming up empty. I have no leaks in my lines but my mc is soaked with brake fluid. Does this mean my reman mc is bad?
 
I have seen porous castings that weeped but only one that I know for sure. Did like you say, just soaked the outside.
 
Ditto Robert.

Also the seal at the rear of the mc is likely leaking, and fluid has contaminated the vacuum booster. Separate the two units and take a looksee.

Your fluid is going somewhere, that is a closed system, it can't just "evaporate"!

I hate "reman" master cylinders, I quit using 'em more than 20 years ago since ya can't make any money doing shit like master cylinder replacement twice on the same job! It's hard enough these days to get a new one that's worth a shit!
 
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