1996 F350 Soft Brakes/No Brakes

[sixtires58]

I'm posting this for a friend of mine, so here's the story. He has a 1996 F350 Powerstroke crew-cab dually that he uses to tow a 5th wheel. Lately, the brake pedal has felt soft and not very inspiring when trying to bring the trailer to a stop so he replaced the master cylinder. After replacing it and bleeding the brakes, the brakes go to the floor. He could see bubbles in the rear cylinder port when the brakes were pushed so they were rebled. Then, he replaced the master cylinder and rebled. Now, the vehicle has been towed to a repair shop and a third master cylinder installed and the brakes power-bled. They called to say it was ready TWICE and each time before he gets out of the lot the brake pedal is on the floor. They keep asking for more chances to fix it. He has no leaks anywhere and says he has good vacuum on the booster. As I said, it's had 3 master cylinders and has been worked on several times by a reputable shop and they are scratching their heads. Any ideas I can pass on? Mark

[danr1]

New master cylinder not a reman, and likely gonna have to replace the front calipers.

Sounds exactly like what my 95 was doing to me out here couple weeks back.

Check the rubber hoses for bulging but look at the calipers to, see how much they flex when under pressure.

[Ohio Ford Farm]

I have not worked on one that new sorry.. if drum rear brakes in the rear check to see how tight the rear shoes are. If the adjusters have froze up and stop turning it will give you a low peddle once the shoes ware down.

And as said look for bubble in a flex hose or a caliper flexing to much.

[rikard]

On my 83 the brake flex hose on the rear axle was ballooning causing a soft and low pedal. I had already replaced the master cylinder and booster because the MC rear seal had leaked, (look for peeling paint on he booster below the MC) and this happened about a week later. I replaced the front hoses too and the difference was amazing.
regards
rikard

[Lazy K]

Ask the shop if they are bleeding the RABS module (if fitted on the frame rail). Otherwise I would would think the RABS module may be bad.

[danr1]

Originally Posted by Lazy K

Ask the shop if they are bleeding the RABS module (if fitted on the frame rail). Otherwise I would would think the RABS module may be bad.

Yea that was the first thing I did, I was fairly sure before I even started working on it based on the way the brakes acted the rabs valve was the problem.
So I picked up fittings and a short section of line to bypass it first thing. I replaced the master after the rabs valve didn't prove to be the cause.

Picked up a couple of proportioning valves for five bucks at the bone yard down the road, no help either.

The new master cylinder pumped up just fine, the remans did not. First one not at all.
It is important the instructions in the box are followed exactly or its not going to work.

Fill half way or so with fluid, use a punch or rod to stroke piston by hand up to 1" but no more then that until the piston feel is solid and only moves 1/8" to 1/4". Then put it on the truck and bleed the system.

The calipers where in good shape and didn't leak a drop, yea had surface rust em but the rubber and slides where in good shape, replaced them and the problem was gone.

I could see them flexing a tad under pressure but can't say the replacements didn't flex less.