weird brake issue

[kevinsranger]

ok guys, i still can't figure this out, here is the story. my rear brake line blew out (rotted) so i replaced the entire line from the master cylinder, to the wheel cylinders (also replaced wheel cylinders). i bled it really good, got a really firm pedal, but as soon as i started the truck, i lost all pressure.

so i replaced the brake booster and master cylinder, rebled the system, nice firm pedal, then i start the truck and bam, lose all pressure. what can this be?

i can plug the rear port on the master cylinder and drive all day long on just front brakes. the only thing i can think of is i have a leak, but i dont see any signs of leaking on any fitting.

does anyone know what can cause this? does anyone know the style flare for the master cylinder line? the one i pulled out (original) looked like a single flair

 

[martin]

If you are not losing fluid I would suspect an internal malfunction in the rabs valve. Try unplugging the rabs valve and see what happens.

 

[kevinsranger]

If you are not losing fluid I would suspect an internal malfunction in the rabs valve. Try unplugging the rabs valve and see what happens.

sorry i forgot to mention i bypassed it, just routed the new line around it. do you think thats the problem

 

[Surefoot1970]

How do you know you are losing pressure? I would take a better look at the self adjusters in the back. They also contribute to pedal height.

 

[kevinsranger]

How do you know you are losing pressure? I would take a better look at the self adjusters in the back. They also contribute to pedal height.

the shoes are new, including all hardware, the wheel cylinders are brand new, and they are adjusted properly, basically i bleed and bleed and bleed, and get a rock hard pedal with the engine off, then as soon as you start the truck, the pedal goes all the way to the floor.

it did this with the old master cylinder/brake booster, and with the new one. but like i said, if i cap off the port for the rear brakes at the master, the fronts work perfectly

 

[Guilius]

if you bypassed the RABS, you might want to put a proportioning valve along the line. sounds like the loss of pressure (and solution by blocking the port for the rear off) is proportioning.

 

[kevinsranger]

if you bypassed the RABS, you might want to put a proportioning valve along the line. sounds like the loss of pressure (and solution by blocking the port for the rear off) is proportioning.

i was thinking that at first, but the more i read on here, alot of members have bypassed it without these symptoms, i just dont get it, and neither does anyone i ask

 

[Guilius]

ive had my suspicions on this. my 1995 Ranger has the 4WABS with the Hydraulic Control Unit.

after an event two weeks ago we are about to do some major modding....3 inch body lift, regear, rear locker, and i blew out one of the front brake lines on the event so we will be redoing all the lines and bypassing the ABS.
no one could tell me if the HCU does the proportioning, and i cant wait to take the damn thing out and apart to know for sure.

i found this on Summit. it may be pricey, but i think it will be WELL worth it.
http://www.summitracing.com/parts/SUM-G3910/?rtype=1

i would at least try a proportioning valve. they are not too expensive and easy to add into lines, and worse case scenario is you spent 30 minutes and it still doesnt work.

 

[martin]

Are you sure the rabs valve is completely out of the equasion? I by-passed mine also and have no issues. I do seem to remember there is a return line from the valve that goes somewhere but it was a long time ago and I don't remember. It has to be completely hydraulically isolated from the rest of the system.

 

[kevinsranger]

Are you sure the rabs valve is completely out of the equasion? I by-passed mine also and have no issues. I do seem to remember there is a return line from the valve that goes somewhere but it was a long time ago and I don't remember. It has to be completely hydraulically isolated from the rest of the system.

I do think there was at least 3 lines going into the valve, i just bypassed the one in and out, maybe that's my problem. but its still weird that i get pressure before starting the car, i have to check that out.

 

[kevinsranger]

Are you sure the rabs valve is completely out of the equasion? I by-passed mine also and have no issues. I do seem to remember there is a return line from the valve that goes somewhere but it was a long time ago and I don't remember. It has to be completely hydraulically isolated from the rest of the system.

what did you do with your "return" line? did you just crimp the line off?

 

[martin]

When the ignition is off the valve internals are in a different position than when the power activates it, I think.