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

[Kurt Nolte]

Huh. Maybe it is a bus vs truck thing. I have been involved in replacing slack adjusters on these buses before, as well as suspension air valves, and there was only the one set of brake components at each wheel: brake chamber, slack adjuster, s-cam, springs. The same rods, arms and cams served for e-brake and service brake actuation.

According to the shop literature, direct action air brakes are an outdated system, so I dunno. I know that the "applied pressure" gauging function for the BCU is the effective braking spring pressure in the chamber...

-Kurt

----- Reply message -----
From: "Tony and Lillie" <tonyandlillie1 at earthlink.net>
Date: Mon, Jul 26, 2010 4:49 pm
Subject: [Vwdiesel]Was: brake bleeding hell on a VW bug-Drumbrake	overheating
To: "Kurt Nolte" <syncronized_turbo at yahoo.co.uk>
Cc: <vwdiesel at vwfans.com>


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