1991 Ford F-350 7.3L Diesel - Braking issue

[unbekannt]

I have a 1991 Ford F-350 7.3L Diesel.

When the brake pedal is pressed, even slightly pressed, the wheels lock up and skid.

When the brake pedal is released, all is fine.

This has had me scratching my head for a minute now.

 

[JayTheCPA]

Which wheels? All?

 

[unbekannt]

To my knowledge it is all. I have watched it brake on gravel and all wheels seem to lock up. At least all the back ones. I will run the test again, braking harder, to determine if it is all four wheels.

 

[Will L.]

Avoid throwing parts in before diagnostics. A new defective part makes things harder to figure out- and brake systems are the worst for most people once this happens.

Pull the wheels and examine the brakes.
Oil or brake fluid sometimes even water soaked into brake material can cause them to grab- the assumption is they would work less but when not totally saturated they soften the material and cause a little swelling and grabbing.

Check Calipers that the slides are not rusty or even muddy- clean and lube them.
Drum brakes, makes sure nothing is hanging up inside especially self adjusters or weak springs.

It could be an abs issue failing- iirc by 91 they were all abs to rear axle?

 

[ak diesel driver]

Any brake work done recently?

 

[WarWagon]

If you can control the brakes enough to try it on gravel roads?! Is this safe to even operate to go out and "test"?

Oil leaking onto rear drums?

Hydroboost pinched return line or filter wrongly put on that line vs. steering box return line. Any restriction on the hydroboost return line and it can't release properly or act funny.