Rear Disc Swap Q's
 

I have a 1994 Ranger that I converted to rear disc using the North Race Cars kit. Now I have a small problem and some technical questions.

The problem is that I have around 1/3 to 1/2 dead pedal travel with the engine running (power assist), but I can pump them up tight with the engine off. Is this normal (for those that have converted theirs to rear disc)? And before you ask, I bled the brakes properly and there shouldn't be any air left in the lines. Also, all the lines, hoses, and brake parts, front and rear, are new. (I also did an Explorer ST front disc swap using 95-97 spindles).

Now for the technical questions. I've read that there could/should be a 10lb residual valve in rear drum setups, usually mounted in the MC. So, first off, is there a residual valve in my stock MC that needs to be removed?

Second, will the stock RABS prop valve be enough to keep the rear discs from locking up, or should I install a second manual prop valve?

Third, would it be a good idea to install a 2lb residual valve in the rear brake line? (Assuming there isn't already a 10lb valve installed somewhere that cannot be removed.)

I may have answered a few of my own questions by looking here:
http://www.therangerstation.com/foru...ead.php?t=4553
Especially the third post.

Greetings, Daniel;

I'm assuming you are using SN-95 rear calipers. Try setting and releasing the parking brake several times. This should take up any slack in the rear brakes. Get back to me and let me know.

Don

Ok, I've set the parking brake several times, bled and re-bled the brakes several times and I'm still not confident in the pedal. I even made a pressure bleeding setup using an old MC cap and 20psi from my air compressor. Still mushy.

With the engine off I can pump the brakes and get about 1/4 pedal travel before stiff. With the engine running the pedal travels past half way before getting stiff.

I replaced the MC. (Yes it was bench bled.) I even swapped out the internal springs from a 95-97 Explorer MC.

Here's what I did tonight. I put the rear end up on jack stands, then started the truck and put it in first gear. I let out on the clutch and let the rear wheels spin. When I stepped on the brakes, the engine bogged down a lot but didn't die, then the pedal slowly went to the floor. All the time the rear wheels never stopped spinning.

This got me thinking and wondering if this was normal for RABS? If the ABS unit isn't sensing much speed, could the RABS valve only be letting a small amount of pressure through to the rear brakes? Still doesn't explain why the pedal went to the floor though...I'm stuck.

Also read where you can bleed the RABS valve. Perhaps the valve needs bleeding or its bad...?

RABS only kicks in when the rear differential decelleration exceeds the programed maximum when the brake pedal is pushed, so I doubt that's the problem.
Try pulling the fuse for it, and see if that makes a difference.

The problems could be in your brake hoses, also. Try clamping off the hoses one at a time, to see if that makes any difference.

Are your calipers sliding properly, or merely bending the brackets?

You may have to do a pressure check on your braking system.

Let me know how these test out.

Don

I guess I need to mention that I did an Explorer ST front rotor swap and converted to 95-97 front spindles and calipers. So, all the rotors, pads, calipers, hoses, lines and hardware are brand new. Not to say that something still couldn't be wrong with any one of those things.

The rear calipers seem to be working properly and everything stays square to the axle housing. If anything the rear brakes provide only a very weak clamping force. I can still wiggle the rotor with medium effort with the brakes fully depressed.

I'm going to try to bleed the RABS valve this evening and see if I get any air or improvement. Also, I read that when the RABS valve is bad they can leak internally. Essentially, fluid leaks into the accumulator instead of being applied to the rear brakes.

I removed the RABS valve this evening and took it apart...

http://home.comcast.net/~djblack94/RABS/RABS001.jpg
Inside the isolation valve solenoid bore.

http://home.comcast.net/~djblack94/RABS/RABS002.jpg
Inside the dump valve solenoid bore. You can also see the accumulator piston, which is covered in junk.

http://home.comcast.net/~djblack94/RABS/RABS003.jpg
Solenoids and pressure switch. Seals and o-rings are starting to deteriorate. I'm not even sure if these things could move if they wanted to.

http://home.comcast.net/~djblack94/RABS/RABS004.jpg
Coils and covers.

http://home.comcast.net/~djblack94/RABS/RABS005.jpg
Black sludge that is covering the inside of the valve.

http://home.comcast.net/~djblack94/RABS/RABS006.jpg
Inside the accumulator piston bore. You can clearly see all the junk that is littering the inside of the valve.

http://home.comcast.net/~djblack94/RABS/RABS007.jpg
Accumulator piston and more junk. You can also kinda see the cracks in the o-ring. This piston was completely seized in the bore and I had to pry it out with a flat screwdriver.

I'm amazed after seeing this that I was able to get any fluid at all to the rear brakes. I'm almost positive this is my problem and why I have no rear brake pressure. Looks like this one will be a core.

Good detective work, there. If there is that much junk and contaminated fluid in your system, you had better give it a good flushing. No point in leaving any of that crap to mess up your new calipers.

Don

Just a side note, you changed the total volumn requirements for the entire system by swapping front calipers and adding rear disk calipers...your pedal may always be soft until you swap to a larger volume capacity master cylinder

I already did the math on that and found that a 1" MC should work fine.

I'll use a 95-97 Explorer with 4-wheel discs as a comparison.

The front calipers are 95-97 Ranger/Explorer, so no difference. Same part numbers.

The rear calipers are SN95 Mustang with 1-1/2" (38mm) diameter pistons. Explorer rear pistons are 1-7/8" diameter.

A Ranger MC has a 1" bore and Explorers use a 1-1/16" bore.

If we do the math, you have a 11.5% decrease in volume out of the MC, but a 36% decrease in volume needed at the rear calipers.

The line pressure will be slightly higher than an Explorer though, because the MC piston area is smaller. Approximately 11.5% higher.

Yeah when pre-01 or 02s do rear disc brake swap you have to switch to the Explorer (for 4WABS) or an aftermarket adjustable proportioning valve (RABS or 4WABS) otherwise the rears will lock before the fronts. After 01/02 Rangers have electronic proportioning so it adjusts itself when you disk brake swap.

Right now it would be wonderful if I could get the rears to lock up... then the fix would be simple. Maybe I should have ditched the RABS to begin with, but I really hate manual prop. valves. I have one on my '68 Charger and I don't like it. It never really feels like I have enough rear brakes, but if I increase pressure it locks up sometimes.