Help - Rear brakes freezing on 68 F100 2wd with PDB and Dana

[jakethe68f100]

New to F100 ownership and this forum. I like both!
The truck is new to us. Its a 68 F100 2wd 360 C6 Ranger Camper Special LB with Dana 60 rear, PS, A/C and what appear to be correct factory 68 power front disc brakes (labeled Bendix if I remember correctly). The truck seems all stock and originally optioned, not modified or converted, and data plate seems to correspond to F100 2WD LB model with the Dana and 360 Engine/C6 Trans. Its an enjoyable truck. It appears to have gone through a few owners who installed new parts (eg front brake hoses and F/R pads etc etc) but didnt drive it much or at all as it had bad engine noise. (I found 5 seriously bent pushrods and replaced the engine and trans together. Impressve it ran at all!).

Problem: When truck has sat for at least 4 or more hours, the rear brakes are initially frozen --- but unfreeze quickly once the truck has started moving under power.
Then, for at least the first few subsequent brake applications, the rears will be very quick to lock up with even ight pedal pressure.

Once its been driving awhile, the rears are still a bit quick to lock on a medum application of brakes, but otherwise don't lock up. The tendency for the locking under light to medium pedal pressure decreases the more it is driven in that segment of driving. Park it, and then start driving it again after a few minutes or even within a half hour or so of parking, and the freeze and quick lock arent very present.

So the problem is on initial driving after the truck has sat for at least 4 hours ish, or overnight or a few days... This behavior has existed from the moment we first drove it home, and ever since. The truck had sat for at least 4-6 months before we bought it.

I have replaced the master cylinder with one from NAPA for a 68 F250 PDB 2WD application, and it has not helped or changed the freezing and locking behavior. Fluid level good, and no indications of decreasing fluid that would indicate a leak in the system. It has newish front brake hoses. Factory proportioning valve appears to be in place as original. The truck at one time had to have been installed with a heavy slide-in camper, given the remaining hardware and associated things when we got the truck, heavy wiring, dual battery isolater, etc etc that suggest it had a large camper and also towed something (like maybe a boat at the same time?) given the massive step bumper and additional undercarriage gusssting of it.

Does anyone have any ideas?
I saw a suggestion of possibly a rear rubber hose that might be old and collapsng and not allowing fluid to return, or something to that effect?

[flyboy2610]

I don't know what to say regarding the sticky brakes, but I can tell you that '68 F100's never came from the factory with front disc brakes. That is a conversion.

[ultraranger]

Very likely the flexible brake hoses need replacing. Over time, the inner lining of the hoses will start to deteriorate and collapse. This essentially creates a 'check valve' where fluid enters the wheel cylinders (or calipers if disc equipped), but fluid does not immediately return to the MC, once the brake pedal is released.

If it was my truck, this is where I would start to track down the problem.

It's also possible the output rod of the brake booster is adjusted out too far, which would keep the brakes partially applied even when you are not stepping on the brake pedal. There should be .005"-.010" clearance between the tip of the booster output rod to the bore depth at the back of the MC.

If the brakes are dragging, this would cause them to get hot and and not fully release, if the output rod is out too far.

[1972hiboy]

I don't know what to say regarding the sticky brakes, but I can tell you that '68 F100's never came from the factory with front disc brakes. That is a conversion.

or a dana 60 rear end?

[guhfluh]

F100'S did come with Dana's, just not full floater 60's.

If it's 8-lug, it's not a F100 and it's a full floater Dana 60 rear.

First thing I would do is pull the rear drums off and inspect. Look for extremely worn shoes, any sign of fluid from wheel cylinders OR axle seals and missing or incorrect hardware.

Drums have a tendency to grab very hard after they surface rust overnight or any contamination increases the coefficient of friction between the shoes and drum.