Rear drum specs on 1995 F-250 HD?

GregH · July 12, 2007

Anyone know the maximum diameter of the rear brake drum on this SRW truck? Brakes are 12x3.0.

I've got a low/soft pedal that sinks with constant pressure. Rear brakes are almost completely inefective.

About 10 different shops have been into this one. 5 master cylinders were installed on the truck. I inspected it and found no contact patch on the rear shoes, poor surface on the drum, broken adjuster cables, and the right side adjuster was installed backwards. (How did these other shops miss the obvious rear brake problems??)

I redid the rear brakes, had the drums machined, and installed new hardware. Then I used the power bleeder and got good flow from all bleeder screws. Drove the vehicle, and had no difference in brake performance.

I pulled the drums off again, and found a small contact patch in the center of the shoes. Also discovered that the parts department gave me 12x2.5 instead of 12x3.0 shoes. My drum after machining (outsourced, of course) is 12 5/16" (real precise measurement - had to use a tape measure!).

So, I'm thinking that the rear drums are out of spec - and probably were before turning. So, new drums and shoes are on the way.

But I'm still concerned about the sinking brake pedal. This vehicle is equipped with RABS, and the owner stated he replaced it twice with a NAPA part.

What do you guys think? Please withold unproductive comments on our comedy of errors, please... /forums/images/%%GRAEMLIN_URL%%/poke.gif

Thanks...

Jim Warman · July 12, 2007

Old timey figuring would have machine to at 12.060 and discard at 12.090.....

Does this truck have RABS or 4WABS?

dieseltech · July 12, 2007

check your master cylinder push rod adjustment. I had the same thing a yaer ago, truck was at many different shops and all had put new master cylinders on. Checked and adjusted push rod and all was like new again. Push rod adjustment spec in WSM

GregH · July 13, 2007

RABS only...

Thanks for the specs. I'll put the new drums and shoes on this morning.

I'll double check the pushrod if I still have trouble...

Jim Warman · July 13, 2007

I've seen RABs valves that leak.... To check, remove the big cap on the end and take out the spring..... Get the pedal to fade away once or twice and check to see if the RABs accumulator piston has moved....

GregH · July 13, 2007

Great suggestion!

I installed the right size shoes, and put a new set of drums on it. Brakes were still poor. Fronts will lock during a panic stop, but rear ABS will only activate on wet pavement. And the pedal is almost to the floor before any of this happens.

Re-bled the brakes - no air, as I suspected.

Remove master cylinder and double check booster pushrod length - spot on.

I've got a handful of fittings and some brake line. My next step was to block off sections of the brake system to localize the problem area. I'll look at the RABS accumulator piston travel first.

ktmlew · July 13, 2007

There was a TSB for GM trucks for this related to bad RABS valve. Said to connect lines together to bypass valve and test for firm pedal. Bet it's the same valve...

Keith Browning · July 14, 2007

I installed the right size shoes, and put a new set of drums on it. Brakes were still poor.

Rear brake adjustment is critical on mid 90's trucks, especially DRW applications. I recently replaced rears on a 1996 ambulance and started having flashbacks. It left with a good pedal and returned last Thursday after 350 miles. The rears needed another major adjustment which brought the pedal back up. You mentioned the pedal sinking. Is this a diesel? Are you aware of the "step through" condition that will pull the pedal down if you continue pushing on the pedal?

Jim Warman · July 14, 2007

"Back in the day" we could purchase brake shoes in both single oversize and double oversize.... Part of the brake reline would be "arcing" the shoes to fit the drum.... The arcing machine was a purpose built device and we would remove material from the ends of the linings...

How times have changed....