Brake bleed trouble on 72 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

My72Cheyenne · 07-20-2004, 11:29 PM

HELP!

I've been trying to bleed the brakes on my 72 (disc front, drum rear) and am having trouble. I started at the rear and have had virtually no luck getting any fluid from the rears (either side). I went to the front and they seem normal. Is it a master cylinder problem, proportioning valve, or what? I checked the bleed screws on the rear to make sure they were not plugged. Any ideas are appreciated.
Thanks,
Tom
Bend, OR

Shane65 · 07-20-2004, 11:36 PM

What's new that you have to bleed the brakes? If you have a new m/c, did you bench bleed it. It's very important that no air be in the m/c. Rebleed the m/c if this is the case. Then, open the rear bleed screws and let the fluid gravity feed. The air will come out as this happens. Keep an eye on the fluid level in the m/c. You don't want it to get too low or you have to start over again.The lenght of bleeding time varies with this method. It's not fast but it does work.

Fred T · 07-21-2004, 03:21 AM

I do the bench bleed a little different than most. For one thing, since the fronts are working properly, leave them alone. To do the rear first be sure of which half of the m/c to do, usually the rears on these, but check the line routing to be sure.

With your assistant in the cab, crack open the fitting to the rear brakes on the m/c. I use a rag so I don't get brake fluid on anything. With the fitting cracked open you should bet some fluid out when the pedal is depressed. If you do get fluid, tighten the fitting while the pedal remains depressed. You should be okay to bleed the rear cylinders.

If no fluid comes out the m/c, you have problems with the m/c.

68C15 · 07-21-2004, 07:33 AM

this method does wonders as it get air out faster.
open screw, push pedal, close screw, release pedal, wait 5 seconds, repeat.
high volume-low pressure baby!

My72Cheyenne · 07-21-2004, 08:33 AM

To answer Shanes question, I had the booster and MC off the truck for complete paint job. I've tried traditional bleeding, like I've always done and like has been described here, but still little or no fluid from rear. I even cracked line loose after proportioning valve and got little fluid out of where the lines go to the rear. It's got to be the MC or proportioning valve. Anyone ever have trouble with Prop valve?

JimKshortstep4x4 · 07-21-2004, 08:48 AM

Bleeding the rear brakes can be frustrating! I would start by disconnecting the lines after the prop valve and bleed the prop valve and master cylinder at the same time. I use fittings and short rubber hoses to return the fluid to the master cylinder unless the fluid is contaminated. If you work the pedal slowly , the prop valve won't shift and shut one end off. Usually the prop valve does not present a problem, but it can.

After bleeding of the prop valve/MC is done go to the rear and bleed the wheel cylinders. If you can't get fluid out after a few trys, break the front to rear line at the flexible hose and bleed at that point. It is possible that the flex hose can shut off the flow of fluid and this is a way to check it. If you can get fluid there but not beyond the flex hose than replace the hose before trying to bleed the wheel cylinders.

Good luck!

JIm

07-20-2004, 11:29 PM	#1
My72Cheyenne "Work in Progress" Join Date: Dec 2002 Location: Bend, Oregon Posts: 55	Brake bleed trouble on 72 HELP! I've been trying to bleed the brakes on my 72 (disc front, drum rear) and am having trouble. I started at the rear and have had virtually no luck getting any fluid from the rears (either side). I went to the front and they seem normal. Is it a master cylinder problem, proportioning valve, or what? I checked the bleed screws on the rear to make sure they were not plugged. Any ideas are appreciated. Thanks, Tom Bend, OR

07-21-2004, 03:21 AM	#3
Fred T Cantankerous Geezer Join Date: Aug 2002 Location: Bel Aire, KS Posts: 6,264	I do the bench bleed a little different than most. For one thing, since the fronts are working properly, leave them alone. To do the rear first be sure of which half of the m/c to do, usually the rears on these, but check the line routing to be sure. With your assistant in the cab, crack open the fitting to the rear brakes on the m/c. I use a rag so I don't get brake fluid on anything. With the fitting cracked open you should bet some fluid out when the pedal is depressed. If you do get fluid, tighten the fitting while the pedal remains depressed. You should be okay to bleed the rear cylinders. If no fluid comes out the m/c, you have problems with the m/c. __________________ Fred There is no such thing as too much cam...just not enough engine.

07-21-2004, 07:33 AM	#4
68C15 blood type; Retumbo Join Date: Jan 2004 Location: next to my reloading bench Posts: 10,269	this method does wonders as it get air out faster. open screw, push pedal, close screw, release pedal, wait 5 seconds, repeat. high volume-low pressure baby! __________________ Man rule #77...if you own a 67 stepside with a caddy 472 you will never be in danger of loosing you man card

07-20-2004, 11:36 PM	#2
Shane65 Registered User Join Date: Oct 2003 Location: Ontario Posts: 108	What's new that you have to bleed the brakes? If you have a new m/c, did you bench bleed it. It's very important that no air be in the m/c. Rebleed the m/c if this is the case. Then, open the rear bleed screws and let the fluid gravity feed. The air will come out as this happens. Keep an eye on the fluid level in the m/c. You don't want it to get too low or you have to start over again.The lenght of bleeding time varies with this method. It's not fast but it does work.

07-21-2004, 08:33 AM	#5
My72Cheyenne "Work in Progress" Join Date: Dec 2002 Location: Bend, Oregon Posts: 55	To answer Shanes question, I had the booster and MC off the truck for complete paint job. I've tried traditional bleeding, like I've always done and like has been described here, but still little or no fluid from rear. I even cracked line loose after proportioning valve and got little fluid out of where the lines go to the rear. It's got to be the MC or proportioning valve. Anyone ever have trouble with Prop valve?

07-21-2004, 08:48 AM	#6
JimKshortstep4x4 Senior Member Join Date: Jul 2001 Location: Muskegon,MI,USA Posts: 6,026	Bleeding the rear brakes can be frustrating! I would start by disconnecting the lines after the prop valve and bleed the prop valve and master cylinder at the same time. I use fittings and short rubber hoses to return the fluid to the master cylinder unless the fluid is contaminated. If you work the pedal slowly , the prop valve won't shift and shut one end off. Usually the prop valve does not present a problem, but it can. After bleeding of the prop valve/MC is done go to the rear and bleed the wheel cylinders. If you can't get fluid out after a few trys, break the front to rear line at the flexible hose and bleed at that point. It is possible that the flex hose can shut off the flow of fluid and this is a way to check it. If you can get fluid there but not beyond the flex hose than replace the hose before trying to bleed the wheel cylinders. Good luck! JIm



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