Depress Brake & Clutch Pedal With Pressure Bleed?

I'm planning to make a pressure bleeder by connecting an air compressor to a master cylinder cap with a hose fitting (should work I hope if the cylinder is refilled after each wheel/clutch is bled). Anyhow, was wondering if it would be advisable to depress the brake (and clutch) pedals to expel old fluid from those cylinders?

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I'm planning to make a pressure bleeder by connecting an air compressor to a master cylinder cap with a hose fitting.

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Sure, that should work but you will need to regulate the pressure to around 15psi. You know that, right?

The cap is 45mm size.

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Anyhow, was wondering if it would be advisable to depress the brake (and clutch) pedals to expel old fluid from those cylinders?

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I think I understand your question. You want to pressure the reservoir and pump the pedals at the same time. Is that what you intend?

Well, I tried to reply, but IE blew up on me. DD ahs a good point about the pressure. I do not think that you have to pump the pedals, but I guess done slowly it couldn't hurt.

My main concern would be using the air compressor. Unless you have a really good air dryer in that line, you will be introducing unacceptable amounts of water from the air into the new and very hydrophilic brake fluid.

There was another thread where a guy bought a new very small pump up garden sprayer, and used that to supply the pressure to the cap. It sounds like a cheap way, and I'm gonna try that method when it is my car's turn.

Well, I tried to reply, but IE blew up on me. DD has a good point about the air pressure. I do not think that you have to pump the pedals, but I guess done slowly it couldn't hurt.

My main concern would be using the air compressor. Unless you have a really good air dryer in that line, you will be introducing unacceptable amounts of water from the air into the new and very hydrophilic brake fluid.

There was another thread where a guy bought a new very small pump up garden sprayer, and used that to supply the pressure to the cap. It sounds like a cheap way, and I'm gonna try that method when it is my car's turn.

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Sure, that should work but you will need to regulate the pressure to around 15psi. You know that, right?

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Right - I'll probably just run the compressor long enough to build up to 15 or so psi (rather than fiddle with the regulator).

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The cap is 45mm size.

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Right - I'm planning to use a GM type cap (Help! p/n 42035) which I saw on:

http://www.audiworld.com/tech/misc24.shtml

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I think I understand your question. You want to pressure the reservoir and pump the pedals at the same time. Is that what you intend?

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Not exactly - what I think I would do is suck out the old fluid in the reservior, fill with new fluid, open one brake (and clutch) bleed screw, depress the brake and clutch pedals to expel the cylindar fluid, close the bleed screws & release the pedals. At that point, I would proceed with the pressure bleeding.

Thanks for the input - if you have other comments I would appreciate it.

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My main concern would be using the air compressor. Unless you have a really good air dryer in that line, you will be introducing unacceptable amounts of water from the air into the new and very hydrophilic brake fluid.

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I'm not sure that would be a concern as little compressed air would actually enter the reservoir and it wouldn't be in contact with the fluid all that long. As well, I will probably drain the compressor tank b/f starting and run it up to only 15 psi so I suspect there would be little moisture present. OTOH maybe that's being optimistic....

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There was another thread where a guy bought a new very small pump up garden sprayer, and used that to supply the pressure to the cap. It sounds like a cheap way, and I'm gonna try that method when it is my car's turn.

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Yes - that's where I found what reservior cap to use. I would work fine but I thought I'd try using an air compressor since I already have one handy.