Okay so I need some help with my Dad's 91 D350. In the past year we've replaced: (in no particular order) both front calipers, front soft lines, shoes all around and a new brake booster since the old one went out. The brakes work like they should except for the fact that you just barley press down on the brake pedal and the wheels lockup so I'm not sure whats causing it. Any help/advise is greatly appreciated and I'll do my best to get you any additional info you might want.

 

If you figure it out, let me know. I'd love to have brakes that would lock up with light pedal.

 

Agreed.

 

I have had one brake booster go bad like that. Just enough pressure on the brake pedal to switch the brake lights son and the pedal would suck itself to the floor and put you through the windshield. If its just a rear brake thats grabby it could be a small brake fluid weep at the wheel cyl. Sometimes a very slight rear wheel seal leak can cause rear brake grab as well.

 

dont we all?

 

Did you machine the drums and rotors? if not the drums could be out of round. To check apply parking brake slowly at a low speed, truck should come to a smooth stop. If surging is felt as truck slows then drums are out of round.

 

I forgot to mention it in the first post but we also put new rotors on, and we checked the drums and they are good.
I did some tests on it to see about the brake booster. First I pumped the pedal with the engine off and then held pressure on the pedal and turned the engine on and the pedal dropped some like its supposed to. Then I ran the engine for a bit and held the pedal down, shut the engine off and kept holding the pedal down for another 30 seconds and when I let off the pedal came right back up. From what I've heard its supposed to stay down for that test if the booster is working right, am I correct?
Finally I took the big vacuum line off that runs from the vacuum pumps up to the booster and tried blowing into it but couldn't so then I tried sucking on it and I couldn't do that either, but I don't know if I create enough vacuum just sucking in to move the check valve.

 

i changed master cylinder ,booster ,brakes rotors everything up front but i couldnt bleed one wheel cylinderand i guess that was full of air ,and it would make my brakes as soon as you touched them suck down and almost stop my truck,,it got so annoying while i was waiting on all the rear brake parts that i just plugged the back line off at the rear end and rebleed the whole system starting with the master cylinder,,,now it has perfect 3 wheel brakes,,,cant wait to get the rear redone though,,,

 

Mine grab bad when it's cold and raining. Is your push rod adjusted too tight at the master/booster joint? Does it get any better once the brakes are warm?

 

Does it the same under all conditions and pedal has very little resistance to it. My uncle took the truck on a trip out of state this weekend and tonight one of the lower ball joints went so once we get that fixed I think I may just do a full brake system fluid clean/bleed and see what that does.