Author Topic: Bleeding brakes - What to do with proportioning valve?  (Read 1372 times)

Offline ih8vols

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Bleeding brakes - What to do with proportioning valve?
« on: December 31, 2021, 11:57:24 AM »
I just replaced all my brake lines and about to bleed them. Im hung up on how to do it with a proportioning valve though.

If I bleed them, it will engage the valve, correct?

I have a by pass that I can screw into the top to keep the valve open while I bleed them, but will it introduce air back into the system when I take it out and put the original back in?

Offline JohnnyPopper

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Re: Bleeding brakes - What to do with proportioning valve?
« Reply #1 on: December 31, 2021, 04:56:42 PM »
If you're bleeding both front and rear reservoirs of the master I don't think it will move the valve either way unless you build up too much pressure between the front and rear brakes.

Just have your assistant pumper avoid going Rambo, nice and gentle.  ;)
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline wz7u

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Re: Bleeding brakes - What to do with proportioning valve?
« Reply #2 on: January 01, 2022, 04:26:41 AM »
If the proportioning valve is working properly, bleeding the brakes should set the dash light off from loss of pressure in the side being bled. Once the air is out (and the fluid can develop equal pressures between front and rear on the valve) the switch should self center and all is well. If the valve sticks the light on after you have all the air out, just crack one circuit or the other open, slowly depress brake pedal and when light goes out, holler at the helper to close it off. A badly stuck valve might require both circuits be done in this manner to fix. That is how I remembered this in the pre-anti lock days.

Like Johnny Popper said, don't go ape crap on things when bleeding the brakes. That's how master cylinders used to get ruined, by the piston traveling too far from an overly enthusiastic press of the pedal with no hydraulic resistance. Don't jamb it to the floor when bleeding the brakes and you should be good.

Great question though. If I'm mistaken, someone let me know.
'79 C20 454/th400 "Pearl"

Offline VileZambonie

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Re: Bleeding brakes - What to do with proportioning valve?
« Reply #3 on: January 01, 2022, 08:29:39 AM »
I just replaced all my brake lines and about to bleed them. Im hung up on how to do it with a proportioning valve though.

If I bleed them, it will engage the valve, correct?

I have a by pass that I can screw into the top to keep the valve open while I bleed them, but will it introduce air back into the system when I take it out and put the original back in?

Is this a stock disc drum setup and combination valve? The way you describe it does not sound that way.
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10