Electric-Vaccum booster swap.

[save_me_jebus]

So I recently started having problems with my electric abs system on my 91 cherokee. From reading around the forums most people suggest doing a swap to the vaccum booster system. As my typical luck goes i had to do rear axle seals before redoing my back brakes. i go to pull the axles and the retaining pin in the diff breaks. (harder than shit to find)

So i get the rear brakes including wheel cylinders done, i put in a vaccum booster, master cylinder and a proportioning valve from an 88 in. The M/C and booster were both new(reman) After screwing around with the lines making them go to where they were supposed to go we bled it out, the pedal seemed slightly low and squisshy. As soon as i fired the jeep up the pedal reaaallly went low and squishy, so i tried bleeding some fluid out of the lines from the master, no difference.

One issue i had, my caliper bleeders wouldnt come loose so the best i could do was bleed it through the hoses.... Could that be my issue? or should i have bled it AFTER i ran the engine?

Any other ideas?

[fredjacksonsan]

ALWAYS bleed from the calipers....especially if they're new. If the bleed nut won't come loose, try getting a line wrench (like an open end wrench, but with 6 sides to grab the entire nut, and an opening to get past the line itself).

The squishy pedal is from air in the system...your Jeep will NOT be safe to drive in this condition.

[save_me_jebus]

ya i know its not safe, the things parked untill its fixed. I tried 6 pt socket, line wrench and vice grips.... They look original so its probably time for a new set of calipers. I figured it seemed like air cause i doubt the booster or master are bad. Ill see tommorow.

[fredjacksonsan]

Yeah, if you've gone to those lengths, then caliper replacement is a good idea.

Did not intend to be insulting with the "don't drive" warning...but there are some people that would...

Post back, hopefully the new calipers will be an easy bleed and solve the issue.

[rhandwor]

Spray the bleed screws on the caliper with P-B blaster. Let stand overnight then using a 1/4 inch six point socket try to open. Tap them on the side with a hammer. If they break Advance or Auto Zone sell rebuilt calipers. The re builder will put an over sized bleed screw in to use the core.
When I lived in Ohio I've heated them cherry red with a torch and they always opened.

[save_me_jebus]

So, the pin for my rearend came in today, so i got that put in. I let my boss feel my brake pedal, he laughed at me and told me to pull the master off and adjust the rod coming out of the vaccum booster. That took away alot of the pedal free play i had. We bled all four corners again ( havnt done calipers yet) The pedal is "better" but i seem to have a veeeery touchy pedal and only front brakes. I took it into the gravel parking lot and hit the brakes and the fronts locked up but not the backs. So tommorow im going to get my boss to check it out and see what he thinks possibly try more bleeding.....

Now this is a bit of a stupid questions, i swear ive heard somewhere that the brake lines from the M/C to the proportioning valve have to be equal length.... that shouldnt matter though should it? i meen the fluid is there its just a matter of pushing.

Go figure i fix cars evveryday with hardly a problem..... fix my own and all hell brakes loose ........(get it.... brakes....)

[rhandwor]

While I replaced all the lines on older vehicles I haven't done newer ones. I live in an area that doesn't use salt on the roads. I've just went to a scrap yard and purchased a piece in good shape. The calipers will have air in them I made a mistake when I was younger and put them on the wrong sides and I couldn't get them to work properly until I switched sides.
Fix known problems and then think about other problems. A lot of times a low pedal is the rear brakes need adjusted or do you have 4 wheel disk brakes.

[save_me_jebus]

So, today i got the bleeders loose bled through the front bled through the back again, no difference. Put in my old proportioning valve and bench bled the master cylinder again...... no difference. The rear brakes are adjusted and new, aside from the drums which were new 6 months ago and i just machined them with the recent work.

Im basically down to wondering 1) if theres a difference in brake size from 88-91, i have 9 inch drums my friends 87 have 10 inch drums. mabey it would effect braking.

Or that the master i bought, since i cheaped out and bought a re-man'd one, that mabey its a peice of crap. As i was bench bleeding it i put my fingers over both of the holes and got my friend to push it with a screw driver, half-way through the stroke the back one quit pushing pressure.... and it is only back brakes that i don't have.

[rhandwor]

Pull the back drums and watch as someone else gently pushes on the brake pedal. Make both shoes move I've had wheel cylinders get corroded and only half work.
Put the drums back on if they are working properly. When bleeding are they getting good pressure. I've gotten bad master cylinders but it was evident when bleeding.
With ABS to properly bleed a scan tool sometimes has to be used to cycle the valves otherwise you can't get air out of the system.
Explain exactly what you removed. On some Chrysler vehicles the proportioning valves are somewhat adjustable depending on the load on the springs. I believe you can buy adjustable after market valves.

[save_me_jebus]

What i removed: Electric pump, master cylinder, accumulator and the other junk assosiated with it. So now i only have vaccum booster, M/C, and proportioning valve. No more ABS

The wheel cylinders should be fine there brand new, the pressure coming out of them while bleeding seems pretty much normal.....

[rhandwor]

http://www.aa1car.com/library/2004/ic30444.htm
Use yahoo search adjustable proportioning valve
The 4 wheel site has some for jeeps from $49.00 to $149.00 make sure the pipe thread size is correct. This is important to save time.
Otherwise buy pipe and a proportioning valve from a same size vehicle at a scrap yard.

[rhandwor]

I forgot to say hoses can swell when they get older. With someone else holding their foot on the brake feel the hose for a section that feels like a balloon is in it, you should have 3 hoses.

[save_me_jebus]

So after much swearing cursing spitting and drinking.... i think i may have discovered the whoooole issue.

went to the parts shop today asked them about my M/C the bore size of the M/C for the 88 is 15/16" I then ripped apart the M/C from my truck and the bore size of that is 1 3/16" biggg difference. So i went onto autozone.com and started doign some comparing between the 2 years.

1988 and a 1991 no abs mastercylinder are identical. so im good there
the wheel cylinders i bought ( same as came off the vehicle) have a 3/4" bore

The wheel cylinders for a 1991 non abs system have a 13/16" bore.

So it almost seems i need different wheel cylinders. The thing im wodnering now is caliper piston size, could they be different.

[rhandwor]

The wheel cylinder is usually sized for the size of the shoes. Check if the shoes are correct for the older system check diameter and width.
I've found when changing systems buy a complete system from a scrap yard.
If it stopped correctly before either keep going so everything is the same size or works.
I think an adjustable proportioning valve would balance pressures so both front and back brakes will work. You can also try to slightly adjust the vacuum booster rod. Make sure the brakes will release when done.
Did you check for a slight bulge in the hose?
Remember if diameter is changed you have to pull the axles and put on larger backing plates. If only the width is wider you will need two used or new drums,brake shoes,wheel cylinders,and brake springs. If you do this again go to a scrap yard find the same size vehicle and buy the complete system.
Ask the parts store if the pads are the same size for both units. If not check rotor size they may have larger rotors another expense.

[rhandwor]

What did you finally find that worked it might help someone else with this problem.