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

[Kurt Nolte]

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

----- Reply message -----
From: "Tony and Lillie" <tonyandlillie1 at earthlink.net>
Date: Mon, Jul 26, 2010 2:58 pm
Subject: [Vwdiesel] Was: brake bleeding hell on a VW bug-Drumbrake	overheating
To: "Kurt Nolte" <syncronized_turbo at yahoo.co.uk>, <vwdiesel at vwfans.com>

I never thought about the fluid expanding. I guess taht makes sense.

On the trucks, they use more compressed air to engage the brakes. IE, the pressure goes up as you apply the brakes. This is for the service brakes only. The parking brakes use a spring brake setup, in which a spring pulls the brakes into contact. The air pressure that's applied and stored in the tank pulls against that spring, releasing the parking brake function. So, in essence, it uses the depletion of air to work the parking brake, while it uses the application of air to work the service brakes.

BTW, when I lost the brakes on the truck, it was clearly that there was no contact whatsoever. They slowly went away, but when gone, there was nothing whatsoever. It was nothing like when I've had a severe recuction in braking on the track due to overheated pads and rotors.

Oh, and that was when I was much younger and more foolish. Thenkfully I was at the end of a very long downgrade, and I lived to tell the story. These days, I don't even rely on the brakes when going downhill. Ahhhh, to remember the days of youth and foolishness.........

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


> One thing to remember is that cars use brake fluid, which expands and
> adds pressure (applying the brakes more) as it heats up, while big rigs
> and other air brake vehicles use air (And increasing pressure just keeps
> the brakes from being applied).
>
> Air brakes use the /loss/ of pressure to appy the brakes, so increasing
> the pressure in the line from heat expansion of the air won't apply the
> brakes and lock them up.
>
> -Kurt