97 F-250 Brake problem

[wrenchtwister1]

I need feedback on a brake problem. I relinded the brakes on my dad's 97 F-250 Powerstroke. The problem is a sinking pedal. I've replaced the master cylinder, rebuilt calipers and wheel cylinders, pressure bled the brake system, and the pedal still sinks to within 2 in. of going to the floor. It stops, but it's scary. I am a proffesional mechanic and have never experienced this problem in 30 years. Any feedback would be appreciated.

[dfordski]

Have you already adjusted the slop out of the rears by applying the emergency brake a few times while driving in reverse?

[wrenchtwister1]

Yes I have. The pedal feels "spongy", just like it has air still in the system, But I've pressure bled the system several times. All my years of repairing autos, I've never had a brake job that has puzzled me like this one has. Thank you for your feedback.

[angus10]

I assume since you say you have been working on vehicles a while , that you bench primed the new MC, correct? And if you did , you could be having a brake hose issue, they can be soft and expand when you use the brakes, or the inside could be torn if the calipers were hung by the hose to keep them out of the way. Also the brake booster check valve could be an issue. Or the ABS control valve. Other then that, you might have a defective MC or it went dry while you were bleeding and seals rolled over.

[wrenchtwister1]

ABS is the only thing I didn't check, everything else I double checked. Thanks for the info

[rhandwor]

Sometimes you need a scan tool to pulse the valves to bleed these systems.
A pressure bleeder might work. If your a mechanic try to borrow a scan tool from a friend.

[dfordski]

How far does the emergency brake depress before any noticeable resistance begins?

[wrenchtwister1]

I already have the bleeder and scan tool. Thanks for the info.

[wrenchtwister1]

It travels about 1 inch. Fully engaged about 4 clicks of the rachet system.

[dfordski]

My 87' gave me the ABS dash light until the RABS Valve was bled. I would think any air at all in the valve would trip the warning light. What effect would occur if someone accidentally put the longer shoes in the front position & the short shoes in the back position of the drums?

[wrenchtwister1]

The effect would be no rear stopping power. No ABS light has come on. Long shoe always goes to the rear, I haven't made that mistake in 30+ years, but thanks for the feedback

[dfordski]

See if this helps : Kelsey-Hayes EBC2 system "RABS" for Rear-wheel Antilock Brake System."One of the most unnerving failures that can occur with this system is the loss of pedal when braking. The problem may feel like a bad master cylinder, but it may not be the master cylinder. The real problem may be a bad Electro Hydraulic (EH) valve in the rear wheel antilock brake system.If a small piece of dirt or rust gets into the unit, it may prevent the dump valve inside the EH valve from closing. The dump valve will then leak fluid into the accumulator when the brakes are applied. The misrouted fluid allows the pedal to drop, and the pedal may go all the way to the floor without applying the brakes. No ABS warning light or fault code will be found either because the limited diagnostics on this system can't tell if the dump valve is fully closed or not.

Wagner Brakes recommends the following procedure to find out if the problem is a bad master cylinder or a leaky EH valve in the RWAL system. To rule out a bad master cylinder, disconnect the rear brake line at the master cylinder and install a plug to block off the port. Have an assistant depress the brake pedal about an inch to purge any air from the outlet, then tighten the plug while the assistant holds the pedal in this position. After tightening the plug, continue to apply pressure to the pedal to prevent damage to the primary cup in the master cylinder as the cup moved across the vent port.

If the pedal holds and no longer drops, it isn't the master cylinder that's causing the problem.

Reconnect the brake line to the master cylinder and block off the outlet port on the EH valve. Then apply pressure to the brake pedal again to see if the pedal drops. If the pedal goes down, the EH valve is leaking fluid into the accumulator. The EH valve needs to be replaced. Air trapped in the EH valve can be another source of trouble with the RWAL system. Some RWAL EH valves (the smaller ones) do not have a bleeder screw, which makes the unit hard to bleed if air gets into the lines. The trick here is to loosen the brake line connections at the valve to vent air when bleeding the brakes.

Brakes can be bled in the usual way manually or with pressure equipment. If a pressure bleeder is used, the combination valve must be held open. (And a personal opinion: If it's a keeper, go Synthetic Brake Fluid)

[wrenchtwister1]

Thanks, I believe this is what I needed to fix the problem.

[dfordski]

Originally Posted by wrenchtwister1

Thanks, I believe this is what I needed to fix the problem.

Not that brakes aren't important anyway, but wouldn't you know it had to be your dad's truck. What did you tell him, "Calipers aren't back from the blueprinters yet?" Anyway I hope this does the trick. By the way, if it is the EH gone bad, moisture build up, which gets worse each time the fluid reaches its vaporous state temp. is the cause. Synthetic doesn't have that problem. In fact the only reason it ever needs change is particle build up. If you put it through mico filtration you could keep using it. It's compatable w/regular brake fluid because it is actually petrolium based too. Just 7 refinement prossess levels higher. In fact it begins the composition process in a gasious state. Good time to make the switch.