[Vwdiesel] Was: brake bleeding hell on a VW bug-Drumbrake overheating

[Tony and Lillie]

You are correct in the application of the parking brake. What you describe 
is exactly what happens.

However, the service brakes work on applied air. Drive a truck with the 
additional guage for the applied brake pressure, and you will see this. 
There is a lot of air available at anytime, generally 100-125psi in pretty 
large quantities.

The failsafe is this: If the air pressure goes too low (35psi in the trucks 
I drive) it automatically ttriggers the parking brake to pop out. That 
automatically then dumps the air pressure holing against the spring brakes, 
and applies the brakes fully via the springs. Check it out sometime, and you 
will see what I'm talking about. I drive and work on them everyday, so I 
know the system pretty well.

BTW, this only applies to trucks and trailers manufactured after 75, when 
spring brakes became mandatory. Trucks and trailers made before that may or 
may not have this feature. In the military, I drove trucks that did not have 
spring brakes. That is also where I experienced the brake failure, in a 74 
International or Freightliner, can't remmember for sure which. I had a 43 
(think) foot long low boy with a 2 1/2 ton truck on the back.

BTW, I'm not dissagreeing with anything that you have for literature, just 
that this is what I observe every day. Also, this matches the description in 
the current CDL manual's for Texas and Oklahoma.

Tony

----- Original Message ----- 
Subject: Re: [Vwdiesel]Was: brake bleeding hell on a VW bug-Drumbrake 
overheating


> Tony, that is the complete opposite of the way my air brake service 
> summary described brake function.
>
> Air pressure acts against a spring to keep the brakes disengaged. When you 
> press the service brake or pull the parking brake knob, you are reducing 
> the air pressure acting against the spring. This allows the spring to 
> apply pressure to the s-cam, which in turn applies the brakes.
>
> This has the added effect of making air brakes "fail safe," as any loss of 
> air pressure engages the brakes.
>
> Maybe trucks are different, but given how compressible air is I can't see 
> air pressure being direct action brakes. You would need a crapton of air 
> to generate enough pressure.
>
> -Kurt