Brakes - Help understanding my brake situation...

[Ty429]

I recently purchased a 68 Ford F100 to fix up. It has an Inline 6 with a 5 spd manual transmission (M5R2). The kids are out of the house and I needed something to occupy my time. Trying to do as much as I can myself. I'm learning as I go and need some help with understanding why my brakes act the way they do...

I have drum brakes front and rear... When the truck in not running there is free movement, meaning if in neutral it will role if the brakes are not applied. However, if the truck is running and in neutral it will not role. Im not understanding how this is possible. Shoot... I'm don't know if I just made any sense (Explained this correctly)?!

I'm sure there're are question before an answer and I most likely forgot to mention something... Ask away?

Thanks in advance for any help.

Ty

[Jacksdad]

Power brakes? If so, it could be your booster going bad. Do they release immediately when the engine is turned off, or does it take a while? Do the brakes lock up without you touching the pedal, or are they not releasing after they're applied?
One other suggestion - maybe jack up each wheel, start the engine and check if it's one or all that are locking up. It would at least eliminate a flexible hose that's collapsing internally and holding pressure.

[Ty429]

Yes, power brakes... and as for them releasing after the engine turned off, yes... and brakes automatically tighten up when engine is running without touching brake pedal.
I will test each wheel this weekend to determine which wheels are restricting free movement.
Thanks for the information/ sense of direction to rectify the problem.

[MadMike]

Agreed it is the Booster causing the issue, but it may not be the booster that is at fault.
Since the M5R2 was installed there may be some custom bits or fabrication done at the pedal box.
Verify what is factory stock and what has been modified.
Verify there is no hissing/whooshing from the pedal area when the engine is on and the pedal is not depressed.

When the engine is turned off, is all boost assist lost? Assist should still be there hours after the engine has been turned off, there should be still 3-4 full assit pedal pumps.
Are there still 3-4 pedals of assist after the engine is shut off?

My two theories are;

Booster piston return spring is failing, normal engine vacuum directly pulls the diaphragm forward just enough to cause drag on the brakes. Or there may be some other fault, such as the valve inside the booster between the MC piston rod and the pedal rod is stuck causing the booster to function without any driver input.

Or.

The pedal assembly has been modified for the M5R2(if hydraulics were installed) and someone did not properly adjust the brake pedal to booster rod length. It is slightly pushing on the booster just enough to cause it to drag. With the engine off the booster bleeds down and releases the brakes. This should be readily noticeable if there is no play in the pedal with the engine on, but there is play with the engine off. For this to happen the booster is also not maintaining vacuum, either due to a faulty/missing check valve on the booster(engine bay side 90° fitting) or the internal valve mechanism.

Here is a good video on how the booster functions.
https://www.youtube.com/watch?v=0sj5d48mcss

[Ranchero50]

Ditto the booster rod length. Been down that route before, smelt the brakes before needing them.