Technical Brake Booster Binding - Brake Lights On

I've been chasing a PITA brake light problem for about a year now and finally have found the problem. 3 years ago I did a power disk brake conversion on the Poncho (51) using a 7" dual diaphragm booster mounted under the floor. I'm using the original pedal setup and made sure the connecting rod is centered with the booster. Year one no issues, last year I had the brake lights stay on a couple of times so I replaced the return spring on the pedal with something a little stronger and that seemed to fix things. This year its gotten so bad I have to pull the pedal back with my foot every time I brake. Finally got some time to pull everything apart and I'm finding the sleeve inside the booster is binding up and not allowing the rod to fully return ( another bigger spring????). It returns enough that I don't feel the brakes dragging, or getting abnormally worm - just not enough for the pedal to make the switch. When moving by hand you can feel/hear the binding.

Is there any kind of lubricant I can try adding to make this slide a bit easier, or am I in need of a replacement?

Thanks in advance for the help.
Cheers TM

 

The problem is keeping the air entering the booster control valve clean and dry. Production frame mounted vacuum boosters used either very high filtration or remote air cleaners to minimize/eliminate dirty/wet input air. Anything that enters the booster also will get into the engine!
I would try some WD-40 to dry out and temporarily lube the control valve, make sure the filter is clean, then do what you can to keep the area around the booster input clean and dry.

 

btt

 

what type of brake switch do you use....if it is an hydraulic type that could be the problem.....
seems they gunk up with the wrong type of brake fluid.

 

It's still the original mechanical style arm switch under the foot board. It's worked for 60+ years - I can adjust it but that's not solving my problem.


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Where does the air enter those aftermarket dual diaphragm boosters?

 

Air enters the dust boot when the push rod opens the control valve, same as any vacuum booster.

 

That makes sense. For the OP dirt must be getting into the booster since it's mounted under the floor. Most are mounted on the firewall and the dust boot is inside the car.

 

btt

 

Under floor booster on my 40 for 12 years and well over 35,000 miles, no problems.

 

Won't be able to get back at it till the weekend - will update when I know more. Thanks


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