help understand my brake weirdnesses

[knorrena]

I have an '85 bj70. On ice the rear passenger tire always locks up first and sends the cruiser into a tailspin. Its not at all bad really, it only really happens when I am driving aggressively. Yesterday I popped of the wheel and drum to have a looksee. I found that the ebrake cantilever was sticky on the passenger side so I used a little seafoam to lossen it up. I used seafoam b/c it starts off thin, seeps into the truoble spot then dries leaving a small amount of lurbication. After freeing everything up I had a quick look at the operation of everything else and found everything to be fine. I repeated this procedure on the drivers side. I also checked ebrake operation and lubed the pivot point on the axle. I left the brakes adjusted too loosely so could test the effectiveness of the front discs. When I jumped and started it, the pedal hit the floor. THis also happened when I redid my front brakes. Then I got to thinking (never a good thing) that this should not happen. I thought that the front and rear brakes were seperate hydraulic systems: if the rear brakes fail should the front ones not work and vice versa? Is my MC shot?

Karl

 

[beanz2]

My theory is that the rear shoe adjusters are jammed and what was keeping the brake shoes in close approximation to the drum was the parking brake adjuster. Without the shoe adjuster (the star wheel you turn to adjust the shoes) functioning, the automatic adjustment no longer takes place and now the shoes are too far from the drums, needing several pumps to work. I'd suggest removing them, clean, regrease and re-install per instructions on the manual. The systems are separate, but when the rears are not adjusted properly, the pedal can still fall to the floor.

Dave

 

[Greg_B]

Has the proportioning valve been taken out of the rear brake line?

gb

 

[knorrena]

No, the proportioning valve is the original one. In fact my first thought was a malfunctioning proportioning valve: the suspension is bagged and the prop valve "thinks" there is a big load on thus there needs to be more force on the rear brakes. It could be this or my front calipers are sticky, or my mc is done for.

Karl

 

[Jim_Hbar]

Karl:

From your description, the slack pedal is only happening the first time that you step on the pedal after working on the brakes, right?

If that is the case, the following applies:
Drum brakes typically have a residual pressure valve (either in the master cyl, or in the proportioning valve) to hold the shoes against the springs, close to the drum. Disc brakes don't have or need the residual pressure valves, because they don't have any springs to retract them. When you work on the brakes, you cause the wheel cylinders (or calipers as the case may be) to retract further than normal, requiring a few pumps of the pedal to bring the shoes or pads into contact with the disc/drum. This is normal, particularly if you are installing slightly worn components. Just remember to do it before you roll the vehicle....I've given myownself a bit of a scare, rolling out of the garage!

If one side of the rear brakes is tending to lock early, look for a faulty auto adjuster or contaminated shoes on the other side, and check out the front brakes! It may be that that is the only wheel which is working correctlyl!

If in doubt, take it to a shop - brakes are arguably the most important system on the vehicle, and it is a false economy to mess with them, if they are at all suspect.

Jim

 

[knorrena]

Thanks for your comments. The brakes build up pressure. Two or three pumps and they are near the top of the pedal stroke. I am not too good at explaining myself, just ask my wife to confirm. I will attempt to be more clear. There are two cups in the mc, one for pressurizong the back brakes, and one for the front. My thought was that if the back brakes were poorly adjusted that the hydraulic pressure from the front calipers clamping down on the rotor would prevent the brake pedal from hiting the floor. In other words, I thought that the front and back brakes, aside from the reservoir, do not share fluid or pressure, and that in the abscence of rear brakes the fronts should still work functionally the same. COnsider this experiment: disconnect the feed tube from the reservoir to the back brakes and disconnect the tube to the back brakes and leave it open, now apply the brakes. Will the brake pedal hit the floor? Will the front brakes operate normally? I think that the pedal should not hit the floor and that the brakes should operate normally. I thought that I was duplicating this scenario by loosening up the rear brakes. Am I? I dont know now.

Yeah, I am familiar with the residual pressure valves, but the 2-4psi residual valve for the drums should have only a small effect here. My prop valve suggestion was based on gut feel. My bj42 behaved similarly and I never dug into what I think is the problem, but may not really be a problem.

BTW, I really appreciate your taking it to a brake specialist suggestion and related comments. I am attempting to repair it myself. If I cant I will take it in to a local cruiser expert (pro-active auto here in Calgary)

Karl

 

[Jim_Hbar]

Karl:

Like you said before, you could have a bad master cylinder too.

 

[knorrena]

Ok, I figured out what my problem is and its embarrasingly simple. The mc has two cascading plungers to actuate the brakes: the pressure from the first operates the second. The front brakes operate first and actuate the rear. The total travel inside the mc is the sum of the travel of the front and rear plungers. If one plunger offers no hydraulic pressure, i.e., poorly adjusted rear brakes, the travel of the plunger assembly increases. Since brakes use leverage to increase hydraulic pressure, small changes in travel in the mc translate to large pedal travel. Normally the adjuster kicks in and keeps required travel to a minimum on the rears to counteract the action of the shoe retract springs. If the rears are sufficiently poorly adjusted the required volume of hydraulic fluid to expand the shoes exceeds a single pedal stroke and the pedal hits the floor.

Man am I dumb. All these years of schoolin and it took a week to figure out what my "problem" was. Feel free to call me a dumbass.

BTW, thanks for your patience.

Karl