OK, now for the monkey wrench!! Disconnected all vac lines, went for a 3-4 mile test drive lots of braking. Came home with a little brake drag again. So apparently not booster related. All brake lines are new, all rotors, new, calipers and master rebuilt.
So which one did I screw up??!!
Well I guess I’m confused.
You went for a drive with the vacuum lines disconnected so there was no power assist.
And you now are experiencing brake drag (which you never mentioned in the original post)?
Hey Harvey, I’ve been trouble shooting the brakes for a few days. I have had a brake drag issue. I jumped in on this thread, because of the brake diagram posted. And with helpful contribution from you and a couple other fellas, I am understanding the somewhat exotic nature if this system. I was thinking my drag was likely a failure if the vacuum side. Looks now like its going to be mechanical.
Karl, brake drag can be several items.
Most common:
Lack of free play in the m/c rod.
Rubber brake hoses swelling shut.
Small piston in the m/c sticking.
other causes:
caliper pistons sticking in bore
large m/c piston sticking in bore.
and although I have not heard of the issue, I guess the servo piston could stick in the bore or the reaction valve could be sticking.
I am sure I am forgetting something…
Tom
Oh, understand.
Another useful diagnosis, if you can get them to still be sticking when you hit your driveway, is to jack up each wheel one at a time and see if only 1 wheel is sticking, or all the wheels. Only one wheel sticking almost surely leads you to rubber hose swelling for caliper piston sticking.
Another way to play this is, if you have an infrared thermometer, use it to check caliper temps to see if one is significantly hotter than the rest.
I have jacked it up immediately after a run. It’s all four dragging. Not so much that I can’t turn the wheels, but 2-3 times normal drag.
I’m thinking it must be that little piston. I have freeplay at the MC,
I’m wondering how important it could be to have good vacuum in the back of the RV. To force the MC back to neutral and release brakes…
Karl, If you disconnected the vacuum lines from the RV, you took it and the little piston out of play. So if I understand your above post, you’ve already done that test. So no, I don’t think it is the little piston.
Going back to Tom’s list, that leaves MC with a sticky bore. It’s a hard thing to see or test but the rod inside the MC must come all the way back such that the port to the fluid reservoir is exposed or bad things begin to happen. That is why Tom mentioned free play. Not exactly sure how to test for this. You could have freeplay at the pedal and the rod still might be sticking internally. Tom, ideas?
Not a definitive test but if you loosen a fitting on any one of your lines, you should release any residual pressure and the brakes should release, at least for that half of the system.
I’m signing off. Will check back tomorrow.
Sounds like you have to pull the MC and check the condition of the bore and seals.
Harvey, Robin, thanks for the extra minds on this. I think you are right, it’s time to pull the MC. There is play in the master pedal. Nothing I would call excessive, but I wouldn’t say the piston is coming all the way back. At least not to the point where the slack comes out of the actuator rod. Hit the rotors when I got home with temp gun. They were all over 230.
Karl, I’ve slept on it and no new ideas. I think there is little harm is pulling the MC, opening it up, and assessing its condition. I just bought a new one from SNG i.e. they can be had for purchase if it comes to that. I used to try rebuilding them but have gotten to the point where I just buy new if I can.
Karl, in my view, your decision may be influenced by the brakes history. If it has been a while since brake work, I suggest do it all at once. Rebuild/replace all cylinders, calipers, hoses, etc. Otherwise you will be doing it piece by piece over a period of time.
If not, you could look under the dash, pull back the m/c boot and see if the piston is returning all the way.
Tom
Yep, when I rebuilt the MC in the course of the brake restoration, I was ready to just buy a new one then. They aren’t cheap, and I convinced myself to just rebuild it. Now I’m ready to toss it. I need to solve this and move on before I roast a caliper or rotor. I’m just a couple weeks from done now.
Tom, that is the frustrating part. I did the brakes as a whole in the last year. New rotors, rebuilt all calipers, all new hoses, rebuild MC. I suspect my MC rebuild now. I’ll pull back the boot and look.
The whole thing seems heat related. It dosn’t start until I’ve used the brakes for ten minutes or so, then after they cool down for ten to fifteen, they roll again.
Since you just rebuilt everything, I would just fix the problem. check the m/c piston is returning all the way, and you have about 1/16 inch or so gap between the push rod and the piston.
Tom
FWIW- I don’t claim to have complete understanding of the system, but when I experienced identical symptoms a couple of years ago, I tried cleaning the little piston and replacing it twice, with slight temporary improvement that disappeared when things got warm. Only when I replaced the little piston, which I recall has a flexible seal attached to it, was the problem solved. It was an older part and I assume it was a problem with the seal.
Thanks John, Yea, I’ve rebuilt the MC, and I’ve pullet he RV apart and I’ve cleaned the bore, and greased that little piston. It made it better, but only temporary. I’m done, I’m cleaning house. I just ordered a new MC. Pretty sure the issue is in there somewhere. I not young enough anymore to be willing to jerk that thing in and out a two or three times to sort it out. If I’m wrong… well at least I’ll know what it isn’t!
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The heat connection had me wondering if you have the heat shield for the booster in place?