I installed a PB booster and bracket last weekend.
I have the adjustable rod to the pedal.
I had to set up the rod at full short to allow for the pedal height to be closer to the floor.
I checked for brake switch operation, and that works.
Drove the truck 25 miles on the freeway after Installation. Then around town the next week.
Pulled out of a shopping center today. I notice the brakes seems to be dragging. No big deal.
Drive 5 miles, pull in for gas. Restarted, drop in gear, and no movement.
Locked up solid.
Towed home, let it cool. All better.
I obviously built in drag, and it got hot and reinforced that failure to me.
Is there a preferred proceedure to set rod length?

 

Did you use a new master?
4 wheel drums? Discs?
Since I'm first up I'll take a guess at:
RPV (residual pressure valve) in the master is keeping your front discs from returning.

 

The output rod between the booster and the MC is adjustable. If it's adjusted too far out, it will be partially applying the brakes at all times. If this is the problem, it will cause the brakes to steadily heat up as you're driving. This can lead to the brakes locking up until they cool back down.

 

Drums all around.
Old mc.
Looks like I need to back off the mc-booster adjustment rod.

 

A bad booster will also show the same symptoms... it could be failing to release the vacuum and only after time has elapsed will it back off.

.

 

I would have loosen both lines at the master to release the pressure and then drive it on home slower tho. .
Orich

 

I pulled the old MC loose from the booster.
Noticed it has a small leak on the booster side.
Now to replace the MC.
During PB install, the tech mentioned the MC-booster adjustment rod was too long.
He removed the adjustment sleeve extension (mc contact side).
The booster-pedal adjustable rod was also too long.
He trimmed 1/2" from the booster-pedal ring bolt.
I tried 5 parts houses to check if there was a MC for PB. Not found. 1 MC fits all 67s.
I will trim 1/2" from the booster-pedal shaft coupling nut and see if that is enough to allow the threaded MC-booster rod to fit.
If that fails, it will be time to buy a matched MC/booster assembly from a parts house.

 

Not all M/C have the same inner piston travel. Rockauto did have some listed the last time, I was on there web site.

Some have a 1" & some have 1.1/8" rod travel.

The M/C piston bore size also can screw ya up to, bigger is not always better if you use one that's to large of piston size you may have a soft pedal with about haft the pedal. Once, I changed to a smaller piston size my pedal was back up to full pedal again..

Orich

 

Actually the opposite will happen. The larger the MC bore, the harder (more firm) the brake pedal will be and the less forward travel the brake pedal will have. It will not have as good of feel for brake modulation and it will also produce less output pressure to the brakes for a given amount of force applied on the brake pedal.

A smaller MC bore diameter will have less firm of a brake pedal, more forward travel but, a better feel for modulation and will produce more output pressure.

This all assumes that all the air is evacuated from the brake system and the drum brakes (if there's drums on the rear) are adjusted out and the output rod of the brake booster to the MC are set correctly.

 

If the MC is leaking out the back end and IF the brake fluid has leaked from the MC and over into the booster, it's just a matter of time before the brake booster will fail.

Brake fluid and rubber booster diaphragms are not a compatible mix. The brake fluid will break down the rubber of the booster diaphragm and cause it to rupture and fail.

Standard practice, if a MC leaks over inside a vacuum booster, is to replace BOTH the MC AND the vacuum booster.

I would not recommend installing a rebuilt MC either. You will be far and ahead to buy a brand new MC.

 

Got it. New MC that is.
It only dribbeled for 75 miles. Booster was new when installed.
Shortened pedal rod mod done.
Related question on brake light switch...
Shouldn't the switch be normally open (power to one side when not pressed)?
Where does it draw 12v?
Mine has no power.
*EDIT-Found the 20A Emergency fuse blown.*
Would have been nice if the fuse block was labelled stop lamp as well.

 

The brake light switch is normally closed, held open. If not for the fact the pedal is in the static position (brake pedal NOT being applied), the switch contacts would be closed and the brake lights would be on. But, since the brake pedal is in the static position and not being applied, the contacts are held open, keeping the brake lights from coming on.

One side of the brake switch should be hot at all times (Red/Black wire, if my memory is correct). The solid red wire is the switched wire to complete the path through the switch, if the contacts were closed (pedal applied) to illuminate the brake lights.

 

On my truck was the way it happened as, I posted it with having the larger M/C over the smaller piston M/C. But not as you stated.

 

Not disputing that your brakes did what they did but, that's an exception rather than the norm of what will happen.


Brake Technical Information