I have always been more than happy with the brakes on my F250. They are the strongest I have ever felt on a truck, and usually only a very slight press of the pedal would bring the truck to a screeching halt.

But yesterday after driving in the hot summer sun for some time on the freeway - when I went to step on the brakes to get off on the off ramp both the BRAKE and REAR ABS lights illuminated and the brake pedal became very hard and stiff. The brakes barely worked well enough to stop the truck. And now this problem appears every time I take an extended drive. The brakes work fine for a while and then suddenly the lights come on and the pedal goes stiff, and the brakes barely work.

Any ideas? The check valve on the brake booster is fully functional, and when I tested the brake booster by starting the truck for a few seconds and then pulling the vacuum line with the check valve out of the booster, it hissed and wooshed with vacuum so it appears to work (maybe it only works when it is not very hot?)

 

I’m having a similar problem with my ’92 with a 7.3 diesel. The brake pedal is very stiff and I have to push on the pedal really hard to slow the pickup. Brakes don’t work very well. The ABS light is on intermittently, but more often on than not.
I have installed new rear brake shoes, bled the lines, installed a new master cylinder, bled again (including the one on the frame under the driver’s seat), vacuum pressure at pump is okay.
Any ideas for us?

 

You should have around 18-20in of vacuum from the pump. If you have that and the brakes are still hard then either you have a massive vacuum leak or a bad booster I would think.

 

Cambiocorsa - I don't mean to hijack your thread.
Lazy K - I checked the vacuum pressure a few weeks ago, but it was in that range. How do you check the booster?

 

Pull the vacuum pipe off of the booster and put the palm of your hand on the end of the pipe with the engine at idle, if no vacuum leaks it should feel like your hand is being sucked into the pipe. If that is the case, with the brakes being hard then the booster is the prime suspect.
If the vacuum feels weak with your hand on the pipe then you should inspect the pipe over its full length right to the vacuum pump and inspect other vacuum pipes for leaks.

 

Yes, my hand feels like its getting sucked into the pipe/vacuum hose. Would a bad booster result in my ABS light on?
I tried something you posted on another thread about pulling spring out of the RABS module. It didn't help with the feel of the pedal or the ABS light.

 

From everything you have said it would seem that you have a bad booster. I would have thought it unlikely that a bad booster would turn the abs light on but anything is possible.
Removing the spring from the RABS module is only a fix for a sinking brake pedal and only when caused by a faulty solenoid valve in the module. So does not apply to your issue.
The RABS only works on the rear brakes anyway (and not very well) so will not prevent the front brakes from working nor prevent the booster from giving you full power assistance.
Make sure that both the rear brake lights are working properly, make sure that the brake fluid level is good and make sure any wiring connections at the master cylinder are clean and tight.

 

It could be a lot of things.

Here is what the shop manual says:
Lack of Deceleration During Medium/Hard Brake Applications

Possible Source(s):
Leaky dump valve.
Stuck shut isolation valve.
Short/open in frame or instrument panel harness (Circuits 519 and/or 523).
-- At anti-lock electronic control module connector (Pins 3 and 10).
-- At RABS sensor connector.
-- At bulkhead or other interconnection.
-- Pinched wire or worn insulation due to improper routing.
-- Test connector (in engine compartment).
Metal chips on RABS sensor.
Borderline high RABS sensor to sensor ring air gap, excessive sensor ring radial runout, damaged or missing RABS sensor ring teeth.
Low vacuum switch inoperative (diesel only).
Hydraulic leak in brake line tubing or rear brake hose, fitting, brake master cylinder, rear wheel cylinder, or front disc brake caliper.
Air in brake system.
Brake related concern:
-- Little or no vacuum boost.
-- Stuck or inoperative wheel cylinder or caliper.
-- Pinched or crimped rear brake hose or brake line tubing.
-- Ineffective brake shoe or pad linings.

 

I put a new booster on this moring. No fix, actually worse. The pedal is really soft and I push on it hard until it reaches a firm point - not quite to the floor, but still very slow to stop the rig.
I suppose my next step will be to look at the rear wheel cylinders.

 

You need to make the adjustment before installing the booster.


/

 

You are now describing a slightly different problem. Excessive pedal travel and poor stopping power.
For excessive pedal travel make sure the rear brakes are fully adjusted up.
If the brakes are fully adjusted up but still excessive pedal travel then probably air in the system. You will need the bleed them in sequence, right rear, left rear, RABS module, right front, left front.
If the pedal is then good but the braking poor pull the rear drums and check that the wheel cylinders are not seized. You can do that by pushing each piston inwards with a screwdriver, should move easily. If any are stuck replace the wheel cylinder.
You can check the front calipers by taking each one off and checking that the piston moves in easily with a C clamp and that the slider pins are free.

 

I thought I'd give an update. New rear wheel cylinders and no improvement. ABS light comes on once I hit the brake pedal and poor stopping. I'm back to trouble shooting...

 

After much work, the problem was solved with a new RABS valve. Thanks for those who gave input!