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I have a 97 F250 CCSB 7.3L that I am building, I have taken axles off of a 2001 f250 and installed them on my OBS so that I can have full disc brake and Super Duty leaf springs.
The OBS came with twin I beam in the front, disc brake in the front and drum in the rear. I am having brake issues now. I have replaced everything in the brake system that I know of, master cylinder, brake booster, RABS module, all new steel brake lines, all new rubber brake lines, calipers, rotors, pads and I have ran about 1gal of brake fluid trying to bleed the system and still do not have proper braking (bled the RABS module, right rear, left rear, right front, left front and RABS module again using the “pump, hold, bleed” method. I have also used a mighty vac to suck all the air out of the system from each caliper. When the truck is off and I bleed the system I have a good hard pedal, then as soon as I start the truck the pedal goes to the floor. After pumping the brake pedal with the truck running the pedal becomes hard as it is supposed to, but as soon as you press the pedal it slowly leaks down to the floor. Today while moving it across the lot, I applied the brakes and the truck stopped but the pedal did not come back up. This does not happen every time. I am running out of ideas, what am I missing? Thanks!
Just out of curiosity, are the '97 and '01 anti-lock brake sensors and receivers compatible? Are they even expecting the same signal?
I don't know that the ABS system would have anything to do with your issue, but I wanted to ask because it seems like that would be an issue.
Yours sure sounds like a mis-match between the booster rod and the back of the master cylinder's piston. Did you check, verify and adjust that gap if needed?
Next I would suspect the new mater of being faulty. It's one of the simpler components on a vehicle, virtually the same design concepts as 70 years ago. But the new parts we get are very often crap right out of the box, being so poorly made, OR defective parts simply put back on the shelf, that the incidences of receiving bad parts is absolutely rampant!
All that means is that stuff you used to assume was good because it was new, falls under the "false assumptions" heading now. Hopefully that's not your issue, but it's something to think about anyway.
The booster is new too? It sounds initially like it's acting normal, but that not coming back up is an odd one.
Are you using a master from the '97 style, or the '01 style?
Sorry I don't have any sure-fire cure from experience. Just some angles to look at. Definitely re-check the gap if you have not already though.
I don't know what the spec is for your truck specifically, but it's a pretty tiny gap no matter what, or the stuff just goes to the floor.
Oh, and did you say that when they work the pedal is pretty firm? If mushy then you might still have air, but if it's firm even sometimes, it's very likely something other than air. Still could be some oddity of the ABS system, but long travel is often that pushrod length thing.
The booster and master cylinder are for the 97 f250, reman master cylinder and reman booster from Napa. All the calipers are installed correctly with the bleeders higher than the brake line. The 2001 axle has front ABS sensors and I do not have them connected to anything.
Your going from a RAB truck to an all four wheel ABS SYSTEM I believe. Did the 2001 donor have a four wheel control unit or a RABS VALVE?
What I am thinking is there is trapped air in the system somewhere, whether in the caliper(s), the MC and/or whatever control valve you decided to use. Like he man asked, did you bench bleed the MC and are you sure they gave you the correct one? If you assembled the cylinder and booster, did you check push-rod adjustment? Did you check the booster for proper operation including vacuum supply?
Just picked up a new booster today for the OBS. Could my issue possibly be that my 2001 axles are disc brakes and my OBS has disc in the front and drum in the rear? Should I be using a booster and master cylinder from a 2001 truck?
I would think that upgrading an older vehicle with a later vehicles more large/modern brakes, the later MC/BOOSTER would have to go with it. Was the original booster vacuum? The 2001, vacuum or hydro-boost?
Did the booster show signs of failure? Did you bench bleed the first MC?
My concern would be adapting from a RAB vehicle to a four wheel ABS System. You retained the old rear? Was the tone ring on the RG or rear drums? I guess the new rear rotors have tone rings. You would then need a period correct RABS valve for disc/disc wouldn't you (leave sensors off front rotors)?
11-15-2018, 09:39 PM
