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Brake Bleeding


201fireman

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I replaced my master cylinder and booster and moved my proportioning valve up to the top of the frame during my SBC to BBC swap.  Front brakes bled fine.  Rear will not bleed (neither side).  I pulled the line off going into the proportioning valve and it does come out. I opened the line just prior to the rubber hose above the rear diff and nothing. I am assuming I did something that messed up the proportioning valve. Any ideas or thoughts as to what I can try?

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I may be stating the obvious but I suspect when you could not bleed the rear brakes you switched to the front.  I always learned you go to the right rear first and then the left rear, then the right front followed by the left front.  In other words you would start with the wheel farthest from from the master and work your way forward to the nearest.  Again, I may be stating the obvious and you may have tried that but when you couldn't do the rear you switched to the front.  

I agree with imr that the plunger may have popped and needs to be reset.

rob

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Todd,

On my '70s that button has always been on the back of the proportioning valve facing the firewall.  I insert a tapered wedge (I use a couple of door/window frame alignment shims) between the button and firewall to keep the button depressed during the entire bleeding process.  Like you, I was never able bleed the rear wheel cylinders successfully unless that button was pushed.  When you are finished bleeding the system, DON'T forget to remove the wedge or the proportioning valve won't work correctly.

Here's a LINK to a Youtube video that explains how a Chevy proportioning valve works and why it may need to be reset.  He uses a truck valve for his demonstration but the principle is the same.

Another suggestion is to use a vacuum fluid extractor (aka evacuator) on your rear bleed taps.  They come in various capacities, vacuum generation methods and prices but all work basically the same. This method, of course, pulls the fluid from the bleed tap through the brake line from the master cylinder resevoir and into the wheel cylinder so you are not pushing it through with the brake pedal.  If you use this method be sure there is sufficient fluid in the master cylinder resevoir so air is not re-introduced to the line again from that end.  As soon as a solid stream of fluid (no air bubbles) is present at the evacuator you can close the bleeder tap. 

I've also found that it is impotant to follow the order of wheel bleeding that Rob shared in his reply.  Good luck.



 

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