[Guest]

Hey guys, I've been having this problem off and on. When I'm out wheelin just crawling around I lose my brakes on inclines. They just sink right to the floor boards. A couple pumps and they are back to normal. On the highway and city they work fine, I've blead the lines a ton of times and the M/C as well with the 2 tubes going back into the resivior. Any Ideas?

 

[peteyg]

I had a customer with a complaint sort of like your's. It was an early 90's Cadillac, and she was having brake issues. Turned out to be a leak in the vacuum booster, right around the seal where the booster went through the firewall.

Pumping them up would bring the brakes back. There was also a bad proportioning valve on the vehicle, which I believe contributed to the failure of the booster.

Those would be the next places to look. I'd suspect the proportioning valve first. If none of those work, suspect the master cylinder.

Just for grins, try bleeding the brakes again, but do a "gravity bleed". All this involves is cracking the bleed screws open and waiting a few minutes for the fluid to run clear. You'll have to maintain the level in the resevior, but this method works really well if you're bleeding them alone. Only crack one corner at a time.

My '69 Chevy pickup does the same thing, but in this case it's the self-adjuster in the front drums. The left side doesn't work well, and so sometimes I get some really exciting moments. Since you're in a CJ-7, this wouldn't really be an issue for you. Rear drums work a little differently.

Good luck with that!

Pete

 

[027]

In reply to:

Turned out to be a leak in the vacuum booster, right around the seal where the booster went through the firewall.

Pumping them up would bring the brakes back. There was also a bad proportioning valve on the vehicle, which I believe contributed to the failure of the booster.

[/ QUOTE ]

HHUUUMMMMM! this is a very curious statememnt, could you please enlighten me on the workings of the booster? You and I seem to have a very different perception on how one works

Oh it was never discussed if this was a vacuum or hydraulic boost assembly I don't think.

In reply to:

My '69 Chevy pickup does the same thing, but in this case it's the self-adjuster in the front drums. The left side doesn't work well, and so sometimes I get some really exciting moments.

[/ QUOTE ]

And just how does that self adjuster in the left front have anything to do with the pedal going to the floor and if you pump it up a few times gets better? Not saying it can't happen, just wondering how it does that.

In reply to:

Since you're in a CJ-7, this wouldn't really be an issue for you. Rear drums work a little differently.

[/ QUOTE ]

Oh really now!!! just how are they different other than the emergency brake?

 

[Guest]

Hey Pete, blead the brakes last night and adjusted the rear brakes they were a little lose.
Hey GP'n it is was orginially a manual system but I put power assist on it. I had this problem off and on when it was a manual system and it's doing the same thing with the power assist. I'm going to guess that it's the proportioning value, I never knew that those values can go south. I guess I should start with that?.
Thanks guys

 

[327]

I had the exact same problem for a year on my CJ; never did figure it out; I recently did another axle swap, went to four wheel disc, and replaced the proportioning valve with an adjustable one. So far the problem has not reappeared, and I had it on some pretty good angles in Moab.

 

[027]

The porportioning valve may have the answer to your problem, the slider can get sticky, but usually in that case one set of the brakes still work a little. sometimes

. But when you added your booster I assume it was vacuum. Did you use the same M/C? If so you may have a bad M/C letting the fluid bypass the piston seal during heat up of brake fluid. This happens in slow travel and lots of brake use. The fluid gets hot and with expanding metal of different types and leaks happen. This would be an internal leak and not detectable by sight.
If you did not use the old M/C you may still have this problem. I have seen New M/C's out of the box sweat brake fluid right through the casting within a couple days, so anything is possible. The proportioning valve will be expensive in most cases if you go that route. You might want to try an adjustable one. Some guys have had good luck pulling the proportioning valve apart and cleaning them but I don't trust that,

I may not get it back just right.

 

[Guest]

Hey guys, thanks. I'm not into taking important things apart. I have a 85 frame beside my house, I was thinking of switching that one. Thought again, I'm going to buy one. What are they worth? The adjustable ones? Any mail order places you guys deal with?

 

[GeeAea]

Hmmm ..did I miss something in the post ??? I didn't see any indication that there was any loss of fluid. If he's losing pedal ...and has no braking at all ...and no fluid loss ...wouldn't one assume that the MC MUST be the culprit????

Now the angle thingie ....enough extreme angle to have the resevour expose the port to the bore in the MC

(obscure, reaching for ???)

 

[Guest]

You know GeeAea, that could be, I'm not sure, but when I was bleeding them and watching the fluid go down(it goes down very slowly)I noticed that the front brake res. bore hole is right at the back and when I go down steep inclines they fail. So maybe is it bringing air into the system?

 

[peteyg]

GP'n, not sure how to put the edited replies in, but I'll try to explain as best as I understand it (fully admitting that I'm always learning).

If I understand it correctly, the vacuum booster assists the manual function of the master cylinder by applying vacuum to a diagphram that in turn creates a pressure differential between a system of chambers, which assists pedal movement to apply pressure to the M/C. When the booster fails or vacuum is not present, the brake pedal can travel to the floor, or it can become very stiff, depending on the style of master cylinder and booster used. Most M/C systems have an override that will allow the pedal to be pushed (hard) to the floor and stop the car. This was the exact experience that happened to the Cadillac. This particular car also has seperate proportioning valves to control the rear brakes. One of them had failed, and was spewing brake fluid. My guess is that the system went low on fluid, the customer panic braked one time too many, and that led to the physical failure of the booster (there was an actual tear in the seal where the brake pedal pushrod goes through the firewall into the booster).

Most Jeeps I've seen use a vacuum style booster, although according to my brake book hydraulic boosters were very widely used on GM cars in the 80's. I do not know what style he has. My CJ7 had vacuum.

I don't know why the front drum system is different. I was probably talking through my a$$.

Happens occasionally. It could be in my case that the truck was converted from a manual system to a vacuum assist system, but the drum brakes were never designed for that. My handy Haynes Brake Manual says that drum brake systems do not require the same amount of pedal effort that disc brake systems do because of the "self-energizing action inherent to the design of drum brakes", and therefore power assist was not really needed until the advent of front disc brake systems. Perhaps the power assist is overriding a loose adjuster in the drum, causing the brake to occasionally lose tension. If the wheel cylinder had failed, there would just be no braking on that corner. It shouldn't affect pedal travel all that much, or else there would be big puddles of brake fluid under the wheel.

Hope I addressed your concerns

 

[517]

I'd suspect the master cylinder. Severe angle or not, the piston bore is full of fluid at rest and will work fine for a single stroke. If the piston is dropping, somehow fluid is being bypassed or air in the lines being compressed. Considering this doesn't happen all the time, I suspect it's not air. Master cylinders are cheap enough to swap in a new one. There is a longer pressure holding demand on the system when you rely on the brakes to hold you on an incline as opposed to just stopping. Any leakage at the master cylinder would be more critical and show up. Still, this is just a guess.