Rear Brakes Wont Bleed - Chevelle Tech
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post #1 of 66 (permalink) Old Jun 4th, 18, 9:21 PM Thread Starter
blm
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Brad
 
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Rear Brakes Wont Bleed

Finished up a fair sized brake job on my 69 SS. Pretty much everything was replaced or restored except the proportioning valve on the frame.
Started today by bench bleeding the freshly rebuilt master cylinder. All went well with that. Hooked up the last of the new brake lines and then went about gravity bleeding all four.
Was able to get the fronts to gravity bleed but not the rears. So after vacuum bleeding i attempted to bleed them by pumping the pedal. After pumping up the pedal and cracking the bleeder open I was getting a small amout of fluid with some bubbles.Subsequent bleeding attempts produced no fluid when the bleeder was opened. Also when cracking open the rear bleeders (both sides) the pedal stayed firm.
So I thought maybe the piston /shuttle in the propotioning valve closed the rear off. But there is no illuminted brake light and then used a test light which didnt illuminate when I touched it to the connector on the proportioning valve.
Obviously something is blocking the fluid to both rear brakes. Last time car was driven (last fall) everything was working as far as the brakes. I know the brake warning light works as it illuminates when emergency pedal is pushed.
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post #2 of 66 (permalink) Old Jun 4th, 18, 9:42 PM
 
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Re: Rear Brakes Wont Bleed

I would suspect the master cylinder. Maybe you accidently installed a seal backwards?
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post #3 of 66 (permalink) Old Jun 4th, 18, 9:56 PM
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Re: Rear Brakes Wont Bleed

Bad Brad, my 70 had good brakes before the paint. It sat for three years and my Hunny Bunny and me spent three hours in my hot warehouse trying to bleed the brakes. The restriction wound up being the rear rubber hose. Not the easiest thing to change as I already put the exhaust on.

Couple of junkers and a few clunkers.
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post #4 of 66 (permalink) Old Jun 4th, 18, 10:11 PM Thread Starter
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Re: Rear Brakes Wont Bleed

Guys i dont think it is either. The master cylinder was professiionly rebuilt and it bench bled just fine. There is no rubber hose in back. It was replaced by a brand new Russell braided stainless line. When i said everything was replaced, i mean every component except the frame mouted proportional valve and the brake pedal. The master cylinder and brake booster were professionally rebuilt.
Something is blocking the path of the fluid to both rear brakes. I think it is letting fluid trickle through. Guess I am wondering if proportional valve can still have shuttle shifted and not illuminate the light. Otherwise thers is something phisically blocking the line from the brand new line hooked to the master cylinder to the split on top of the rear end.
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post #5 of 66 (permalink) Old Jun 5th, 18, 12:23 AM
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Re: Rear Brakes Wont Bleed

SOMETIMES the rear piston in the master does not fully retract due to rod length or a piston with a different rod depth. This can be checked by observing the master with the cover off and ensuring you see two little "geysers" erupting from the balance ports at the bottom of the reservoir.
Many rebuild kits do not have the correct depth in the piston for the rod that is used. If the balance port is blocked by the piston then fluid cannot enter the piston chamber.

Some try loosening the master to have the master move away from the booster to alleviate this but that is only a temporary fix.

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post #6 of 66 (permalink) Old Jun 5th, 18, 12:30 AM
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Gary
 
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Re: Rear Brakes Wont Bleed

Sometimes the valve in the proportioning valve sticks. Check for pressure at the valve
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post #7 of 66 (permalink) Old Jun 5th, 18, 1:07 AM
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Re: Rear Brakes Wont Bleed

I'm no brake guru....disclaimer....I've been seeing this topic a lot lately....It's almost epidemic in scale.....

https://www.performanceonline.com/Co...ve-Bleed-Tool/

"Performance Online's new Combination and Proportioning Valve Bleed Tool is designed to replace the warning light switch (white plug) on the proportioning valve during the brake bleeding process. After the bleeding process is finished, simply remove our PvBT Tool and reinstall the warning light switch. This item makes bleeding your brake system much easier. Works with all popular non adjustable GM style combination valves. Will also work on AC Delco 172-1353, 172-1371 and many other GM proportioning-combination valves.

Tool is designed to lock the low pressure shut valve to allow fast and easy brake system bleeding. No need to constantly remove low pressure switch and center the low pressure valve during the brake bleeding process!"
------------------------------------------------------------------------------------------------------------------------------------------------
Some proportioning valves have a button on it but it only works if the front brakes blow out. The valve has to move the other direction for the rear brakes.

Also improperly adjusted or sized master plunger can cause the issue. When this happens the piston can't move back far enough to clear the supply hole. I'm currently clueless to this adjustment. If you are using stock parts this should not be an issue.

This dude gets to the point.......................eventually.

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post #8 of 66 (permalink) Old Jun 5th, 18, 2:42 AM
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Re: Rear Brakes Wont Bleed

start at the prop valve outlet to the rear and see if you get fluid out of there, if so reconnect and move to the junction above rear axle and so on.
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post #9 of 66 (permalink) Old Jun 5th, 18, 3:49 AM
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Re: Rear Brakes Wont Bleed

https://www.chevelles.com/forums/16-b...oster-pin.html < Chevelle tech tread you should give the once over.



I think your problem is probably in the proportion valve and your wiring to the idiot light or switch itself is bad, Coming up with a test for the switch and wiring would be easy.

You can pull the switch and try to reposition the valve, but if your front brake pressure builds up and the back don't the shuttle valve will move to block the rear brakes again. That's the reason for the bleed tool. I can think of a few ways to allow the front fluid to flow freely and not build pressure while you bleed the rear brakes. A hose on the open bleeder to the front brakes in a bottle with fluid and bleed the rear until no air. If the shuttle moves to block the front, push the shuttle reset button while adjusting the front brakes (if equipped with reset button). I'll leave this to you and the experts. These are just quick thoughts that may set you on the right path.

Videos are just informational and might help.

I included the thread on the master cylinder to booster pins. Different pins and master cylinders. Master can look the same on the outside yet have a different style plunger requiring a different pin or the spacer plug. I've seen this as an issue especially on upgrade kits and mix and match brake setups. If you have the old master compare the plunger face to the new.

We need some Jack Action up in here. I see he's back in action lately. Hope to sponge up some of his knowledge.
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post #10 of 66 (permalink) Old Jun 5th, 18, 8:18 AM Thread Starter
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Re: Rear Brakes Wont Bleed

Al, think you may be on to something. Thanks for the push rod.link. Got to thinking about this dilemma last night and I have a new suspect. The swith/shuttle/warning light on the.prop valve is telling me that all is good, so I now suspect the pushrod on the booster. It is the original booster for my 69 and was just rebuilt. However the master cylinder that I removed was an old aftermarket. The just rebuilt master cylinder is an original GM Chevelle 69 unit. However I don't know what pushrod was/is on the booster. Maybe someone changed that when they put the aftermarket master cylinder on many years ago.
I wasn't really paying attention to pushrod length when I bolted on new master. Maybe the existing push rod is a little-known too long? Going to have to study up on the particulars.
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post #11 of 66 (permalink) Old Jun 5th, 18, 9:01 AM
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Re: Rear Brakes Wont Bleed

I may have had that issue a hundred years ago. I made a tool that will bolt to the side to keep the valve open. I got the brake info from the AIM book.
Check out the AIM for the Brake tool that G.M. uses. I made one like it.
Bob
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post #12 of 66 (permalink) Old Jun 5th, 18, 11:36 AM
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Re: Rear Brakes Wont Bleed

Break the line before and after the valve and see if the fluid stops flowing.

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post #13 of 66 (permalink) Old Jun 6th, 18, 2:27 AM
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Re: Rear Brakes Wont Bleed

I'm glad you picked up a few hints and have a direction to pursue.

You should also have picked up on the differences in master cylinders from looking at them from the pushrod entrance? Rhe pushrod holes may different and some use an adapter to fill the void in the master cylinder.

You can test function of your brake malfunction wiring by grounding the wire at the switch and checking for the light on the dash with the parking brake off, key on. To test the switch you need to pull it.

Last edited by DUTCH MAX HEADWORK; Jun 6th, 18 at 2:57 AM.
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post #14 of 66 (permalink) Old Jun 6th, 18, 11:56 AM Thread Starter
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Re: Rear Brakes Wont Bleed

Well I have determined that all is well with the push rod. Old master is of the deep recess just as the new one is. Next opportunity I have to work on it I think I'll try the following. Close the bleeders except the right rear. That bleeder I'll completely remove. Remove the lid from the master cylinder and just see what happens. I'll give it overnight if I have to. Nothing seems to be amiss with the system. Maybe the problem is impatience with the operator. Anyway I will report back on whatever the fix ends up being.
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post #15 of 66 (permalink) Old Jun 6th, 18, 2:46 PM
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Re: Rear Brakes Wont Bleed

As others have already stated, pump up the brakes and start opening the connections to see where you start getting fluid flow from. By process of elimination you'll know where to look instead of guessing.

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