Hey Guys Have an issue with the New to me 83' F-700 hydraulic Brakes grabbing when pushing on the brake pedal,,, foot pushing on the pedal, goes down about halfway, then Whamo,,truck slams to a stop, son almost hit the dash, tires lock up a bit, and this is just in the driveway.

This truck does not have the Lucas Girling brake system, just a single line system, with a vacuum booster, and a Hand operated parking Brake

They did seem to work fine when I purchased the truck,, until the vacuum check valve split in too, the truck did get driven(just in the driveway) with the bad check valve until we figured out what the problem was, since replacing the check valve the brakes have been grabbing hard, I have bled each wheel cylinder, and the vacuum booster as described in the Tech section.
Any ideas what could be causing this ??,,, Thanks, Mark

 

Any evidence of a leaking wheel cylinder or two? Brake fluid on the inside of the tires perhaps?

 

I have not pulled the wheels and drums to look at the brakes, but there is no fluid leaking down on any of the backing plate or wheels, today rather warm outside about 40 degrees, so we messed with the truck a bit, replaced the Ignition module, got one from Napa,, the truck starts and runs again(out of the Blue, it just would not start about a week or so ago)Now we got to drive it around a bit today, Brake pedal goes down rather far with no braking,, then alittle more and, whamo brakes come on basically locking up, felt the front end lockup and slide on the snow in the driveway.

What happens when the Brake Booster malfunctions ?
Is there only hard to apply brakes or can there be a situation where the atmospheric pressure can slam on the vacuum brakes almost instantly ??

I need to test the Booster somehow ? Any ideas ?

 

Try the brakes without the truck running.
Does it only do this when going forward, or will it do it in reverse.
I suppose this is drum front and rear?

 

It has drum brakes front and rear. I do have a slight incline on the driveway, in the am I will try the brakes with the truck not running, on the incline and see if they still Grab so badly.

Weird thing is early this summer we went to look at a 69' F-600, 4-6 yard dump truck, thing was in decent condition cab and rot wise, when we went for a test ride the owner warned us on how intensely the brakes worked, man this truck was scary to apply the Brakes,,, stopped so hard almost put us in the dash several times,,, if you pumped the brake pedal it was a little better, now my f-700 is doing about the same thing.

 

OK this morning I got the truck into position on the driveway where it would roll, after warmed up a bit, with the truck not running, let it get moving along rolling, foot on pedal pushing down about halfway, brakes pretty much slam on, skidding in the snow a foot or so.

Back the truck up, shut it off, use up the vacuum reserve this time, start rolling going along, push down the pedal, whamo front and rear brakes lock up, with the front sliding sideway a bit on the snow
My son found a booster test, truck off, press brake pedal a few times using up the vacuum, keep foot on the brake pedal, start the engine, pedal should go down slightly if Booster is Good------Pedal Did Not Go Down when engine started !!

Then shut engine off and could hear vacuum leaking in Booster, (my ear right up to the Booster Tank)drained the vacuum gauge down within 10-15 seconds or so---thinking the Booster is Shot !! Also found the Booster Air filter behind the seat snapped off from its mount, and the cap loose, wondering if some foreign material got down inside it, HHMMMM,, Any thoughts ? Mark