Stumped with soft brakes

[Shamoke]

I have a 1988 F-350 7.3 idi and I am having stopping issues. I have had a mechanic work on this truck trying to get the brakes to work properly with the following parts all being replaced.

Rear brake shoes, drums, brake parts, rear inner and outer bearings, rear axle seals and both rear brake lines. Popped open the differential and changed the oil with new synthetic oil.

Front brake pads and brake lines replaced, 2 year old master cylinder was replaced along with the power booster and vacuum pump and are all now brand new. All brakes have been bled three times and the old goose neck trailer hydraulic over power brake system was removed.

There are no brake leaks but the soft braking problem remains. The pedal goes three fourths of the way to the floor before I get any braking at all and what little I do have is not enough to stop safely, it actually feels worse than it was before. The is a warning light on the dash that says "Rear Anti-lock" that has been on since I bought this truck a year ago and we replaced the rear ABS brake valve but the light remains lit.

I can think of nothing else to try here that would cause soft braking, the pressure is going somewhere but where? We have bled and bled the whole system and see no air bubbles so I don't think that's the problem. Anyone else have a similar issue or have ideas of what to check next?

 

[jrollf]

If your Rear Antlock break light is on, check the RABS valve. On my 93 its on the frame rail under the driver's left foot. It can cause soft brakes.

There is a bleeder on the RABS, it must be bled to get air out too.

If the RABS valve is leaking internally around the valve, it will also cause soft brakes.

Some people prefer to use a hardline and bypass the RABS valve as it can cause headaches to get it to work right. But then of course you will no longer have Rear ABS.

On my truck I have also found the brakes can be spongy if the rear drums are not adjusted tight enough.

1993 F350 Crew Cab Dually
7.3L IDIT with a Banks Sidewinder
ZF 5-spd manual transmission

 

[Shamoke]

Thank you very much for this info, I had no idea that valve was there. I'll be having my mechanic look at it and either replace it or just try to bleed it, either way it's good to know I'm not going crazy thinking my brake fluid is magically disappearing when I need it and then reappearing like nothing was wrong.

 

[Cubey]

RABS seems to make no real effect on my F250. The light has always come and gone but it stops fine.

 

[idi_econoline]

I tried bleeding my old 89 F250, and never getting it firm. Towed it to a shop, maybe 12 years ago.

Just replaced the ABS valve, master cylinder and booster a few weeks ago.

Saw a guy on youtube, who swears by removing the spring from the RABS valve before bleeding. I did that this time, with successful results.

Need a 1 1/4' socket. Strangely enough, I had one. Don't know why!

 

[Noiseydiesel]

Before, before you waste your time going back to the mech, drive your truck up to 30 MPH, then make a firm stop. Do it 9 more times. Chances re by the time you get to #8, you will be smelling brakes getting hot. Do it 2 more times to 10 and then let it cool off for 30 minutes. Then go drive it.
Brakes feel normal, right?