1984 Audi 4000S Quattro brake bleeding

Hi, I think there is air in my rear brake lines, but bleeding is a rediculous pain. I know the proportioning valve on the '84 is not adjustable, but is there a way to disable it? I know the first ones on
Jeeps there was a button that could be held. I just want a way to keep normal pressure or lot's of pressure to the rears while bleeding. Right now I can bleed once and then the rears get closed off.
My main problem is that I have either a leak or air in my lines, but I can't see a leak and don't seem to lose fluid. If I press my brake pedal slowly it goes about 3/4 or 7/8 to the floor, but still feels like a normal pedal. When it does this I get pressure to all 4 wheels, but probably about half of what I'd call normal pressure. If I push the pedal more rapidly or fast it will only go half way and feel like a normal pedal with full pressure, but only the front wheels will have pressure. I'm hoping it's just air in the rear lines and under lots of pressure that compressed enough to divert the flow and close off the rears. If it's something more serious then maybe there's a way I can make the fronts always have the pressure? Right now I can brake fine with the mostly depressed pedal, I'm just worried when I go to get the car inspected they will say it's inappropriate that way.
Thanks for any help, Chris
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Chris, The brake system is supposed to be a closed system, so any air in the lines or loss of pressure is a serious issue. On the proportioning valve, it's supposed to reduce the pressure to the rears when the car brakes - as when the nose dives - thus pulling on the spring which activates the valve. In order to bleed the system, IIRC, you have to fool the prop. valve into thinking that the car's suspension is at rest (some of the postings on Audifans mention pulling the prop valve lever to the rear of the car). If you don't trick the valve, you'll only have 20% of the brake pressure going to the rear, and 80% to the front. WRT the brake system, if you're using the original lines then you're asking for trouble. Old lines can become herneated and collapsed internally and you'd never see a problem until they burst. Check the hard lines for excessive corrosion as well - you can get lengths of metric bubble flare tubing at your local FLAPS. The difference between slow and fast presses of the pedal sounds like brake fluid is bypassing the plunger in the master cylinder. I have read postings where people have mentioned that the seals in the MC get damaged while bleeding, as they pass along areas of the cylinder that they wouldn't normally (and where some corrosion may exist) when the pedal is pumped to the floor. Brakes are something that should not be "mickey moused" - they're not optional equipment. You need all 4 wheels to have them, not the fronts only. Cheers! Steve Sears 1987 Audi 5kTQ 1980 Audi 5k 1962 and '64 Auto Union DKW Junior deLuxes (SPAM Blocker NOTE: Remove SHOES to reply)

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I wish I had a spring or a lever. This is an '84. '85 to '87 used the spring and lever to tell suspension diving and adjust accordingly, it was an adjustable prop. valve. The '84 does not have this. My prop valve is just below the master cylinder and completely unadjustable. All of my brake lines are steel and seem to be in good order. I was looking/hoping for that fooling pressure to the rear step you mentioned but for an '84.
My brakes do work as per new if I push the pedal 'slowly' or whatnot, I'm thinking when pushing fast it sense a rear leak and closes off pressure. I suppose I should have been saying combination valve the whole time instead of prop.valve.
Anyway, thanks for your help, Chris
Steve Sears wrote:

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Chris, You might try asking the 4k people on Audiworld, or try Huw at www.humanspeakers.com/audi I have cc'd the q-list at Audifans to see if someone may be able to provide assistance (q-listers, e-mail replies to Chris directly) Cheers! Steve Sears 1987 Audi 5kTQ 1980 Audi 5k 1962 and '64 Auto Union DKW Junior deLuxes (SPAM Blocker NOTE: Remove SHOES to reply)

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