Brake Mystery

[richbenp]

Howdy,

I've got what seems like an intriguing problem. Recently, I lost most of the braking power on my 91 Explorer. Basically, the pedal went to the floor, and I could just barely stop (as I was pulling up into my driveway, thankfully). It might have been creeping up on me before that, but I don't remember noticing. In any event, I first went to bleed air out of the lines (if there was any), and discovered that the bleed screws on my rear drums where rusted in place (the old girl lived through 3 Michigan winters, and they salt the roads, alas). After removing the slave cylinders, I discovered that 1 side of one of them was frozen, so I replaced them both, had the drums turned (they were within spec), and replaced the pads and the springs. Then I bled the whole system. Still the same problem. Brake pedal sinks to the floor. I can't build up pressure by pumping the brakes. So I replaced the master (it seemed like the obvious thing). This didn't fix the problem. I next removed each wheel, and determined that yes, the pads are moving when the pedal is pressed. I manually adjusted the rear brake pads so that the drums are just short of dragging. When I push the pedal, it sort of sighs to the floor, it doesn't fall to the floor, and there is something of a shhh noise (that might be my imagination). The booster doesn't seem to be leaking. Oh, and although I can't get the brakes to grab and skid the truck, so I can't stop very quickly and would be afraid to try it at real speed, if I push the brakes down (so basically to the floor) and step on the gas, the car won't go anywhere, so they are holding at some level. I think I had it up to 2500 rpms, and the truck just sat there (well, she wasn't happy, but she didn't go anywhere).

I've read that you can mess up a master by pushing it all the way in, which I certainly did while I bled the brakes (I bled each wheel until clean fluid came out, so I pumped for a while on several of them). Is there a way I can check this? It seems that since I do have some hold at the bottom, this isn't the problem. Is there something else that might be happening, swelling line, something strange with the RABS (mine doesn't have ABS)? As far as I can tell, I'm not leaking anywhere. The reservoir isn't going down, and there aren't any puddles, though I've been out there pumping the brakes ad nauseam as I check first this wheel then the other.

Any help would be appreciated.

 

[gmanpaint]

Did you bench bleed the new master before it was installed?

Any fluids inside the drums or wheels pooling up?

If there is no leaks, there has to be air in the system. Try a Mightyvac pressure bleeder. $40 at Harbor Freight.

I really have not heard of anyone having problems with the booster, but that doesn't mean they don't go bad and develop a leak.

 

[richbenp]

Yep, I bench bled the master, although some leaked out while I tried to get the lines reinstalled. But I put a whole bottle and a half through the system after it was on, and I don't seem to build pressure by pumping the brakes, so I thought air was eliminated. Perhaps there is some place in the system that air can hide? I'll look into the pressure bleeder.

 

[92exp4x4]

Are you sure you did not let air in before tightening the bleeders? The pedal cannot move after its compressed completely, before tightening the bleeder. If it does it will draw air into the system through the bleeder.

I assume you know this, you seem fairly savy on the subject, but I just want to throw it out there.

There is a bleeder screw on the RABS valve on the drivers frame rail forward of the fuel filter. If you need to, you can take this valve out of the system and plumb around it. It is an option, and will not effect the normal operation of the brakes. You just won't have any rear anti-lock anymore. I did this on one of mine as it was leaking badly. I left it plugged in and zip tied it to the frame so as not to get a light.

I have also had a mushy pedal for a time after doing a complete brake over haul. I had enough pedal to drive the truck safely and as I did so, the pedal got firmer with time. It took about 300 miles of stop and go driving, but it's fine now.

I literally replaced all but the master cylinder, even the rubber pedal pad!

 

[richbenp]

I don't think that I drew any air in, but ... The first time I used a one man bleeding setup and ran back and forth lots of times. My wife wasn't interested in helping out on the old beater, especially since the 8 month old is teething! The second time a mechanically minded friend manned the bleed screws. He did get a bit of air out of one of tires (rear driver side), but it didn't seem to effect the problem at all. Reading some other threads, it seems that a sticking RABS might cause this symptom? I tried to loosen what appeared to be the bleeder screw on the RABS, but it wouldn't budge and I started to wonder if maybe it wasn't a screw after all. There isn't much room in there, but I was about to rip the whole unit off the frame, or so it seemed. I bought today the bits and pieces for a little homemade pressure bleeding, but I think that first I'll plug the outward line from the RABs just to see if I can't isolate the problem. Plumbing around the problem is something I was considering, but wasn't sure if it was safe. I've never had the rear anti-lock come on, despite plenty of snow driving, so if removing that capability is all it does, that seems like a good way to go.

Oh, in all of this I also discovered that one of the front disks is cracked, and the rear limited slip seems to be more or less gone (isn't that what it means if I can turn one of the rear wheels without turning the other?)

All said however, I still love this truck.