Greetings all!

'86 F150, I6, 4X4, 4 speed OD.

Here is the problem: The first 2-3 times the brakes are applied first thing in the morning, all 4 wheels lock-up with only the slightest pressure on the pedal.

Here is what I have done in the past months: New calipers, pads, rotors, flex lines, rebuilt master cylinder, rear wheel cylinders, shoes, turned drums.

Everything has been working PERFECTLY all summer. This has just started happening within the last couple weeks. I flushed the whole system with new DOT 3 on all 4 corners last weekend (not even a bubble in the whole system!) and replaced the brake booster check valve but no change. It seems that after this first couple "lock-ups", even though the truck is still cold, the brakes perform normally.

I am thinking that the brake booster may be failing, but I am not willing to buy a new one on a guess! I also inspected the brake parts and saw no contamination of shoes or pads, or leaking of any fluids.

I did search the site and found a couple threads with folks arguing about adjusting rear shoes and brake shoes/pads being contaminated causing "grabbing brakes" but I have ruled that out already.

Any guesses or opinions welcome and thanks!

HARFEND

 

I would guess the brake booster too.

 

booster is my guess also if you're SURE its all 4 wheels locking up

 

I just thought of something else, rust on the drums and rotors. But I only have this problem if the truck sits for a couple of weeks.

 

Thanks for your prompt replies!

No rust on the drums for sure Franklin, I just checked and everything looks good plus the truck is a DD.

HARFEND

 

More thoughts...

I am wondering if this brake grabbing/lock-up that only happens in the morning or after the truck sitting still all day is a issue that will get progressivly worse or could this be the extent of it?

When I changed out the check valve on the booster, there was a vacuum present in the booster but I had just turned off the truck. I checked the vacuum at the hose and there was plenty enough to hold on to my thumb. (No vac gauge! Had to use the old style method.) If the hose was'ent blocked the motor promptly stummbled as expected.

This leads me to belive that the booster can hold a vaccum and it should build it quickly enough for the brakes to work correctly...?

Is the brake booster a work/no work componant or do they fail slowly over time?

Any thoughts or opinions welcome!

 

I've been having this problem after I put in my new brake booster not too long ago, although not bad enough for me to mind it at all. As for booster failure, it could be either. I replaced mine because the internal rubber diaphragm cracked; the booster worked fine and I still had power brakes, but it would leak air where it connected to the pedal every time the brakes were applied, for as long as the brakes were applied. I suppose it could suffer a catostrophic failure, too (mine never did), but that wouldn't that just cause loss of power brakes? It's possible there's an issue with the push-rod that goes from the booster into the master cylinder, that could possibly cause your brakes to lock up.

 

has it been cold the past couple weeks? and it worked fine all summer (warm)? wonder if there's water in the lines and it freezes over night... have you tried using a power bleeder?

 

There is a warning on the rebuilt boosters you get, not to pull the rod out that goes to the master cylinder. They say a disc inside can get out of wack and cause the brakes to be touchy, though I would think they would be touchy all the time, not just in the morning.

 

your booster should have vacuum stored when you shut it off so I'd say you have an internal leak (new checkk valve, right?) To check pinch the hose shut with needle nose pliers and see if the engine runs different.

 

a long time ago on my first car..i decided to adjust the brake pedal height by moving the pedal rod...
so when it heated up the brakes locked up!

 

Here is an update on the brakes.

Thanks to the question posted earlier, I checked to make sure all 4 wheels were locking up.

Turns out it was only the left rear. A simple rear brake adjustment and I am pleased to report no further surprises hitting the brakes first thing in the morning!!

I guess I helped to prove lots of advice on this site is correct; that when diagnosing a problem, start simple!

Thanks to all who helped sort this out with me!!
HARFEND

 

I'd pull the drum off to make sure you dont have a leaky axle seal or wheel cylinder...possibly e-brake cable but I doubt it.