Front brakes won't bleed
Thread Starter
Pro
Joined: Jan 2017
Posts: 653
Likes: 51
From: Beaver County PA
Front brakes won't bleed
I'm doing a frame-off rebuild on my 72 and replaced all the brake lines from master cylinder to calipers. I bench bled and installed a new master cylinder. I started bleeding the lines with the back wheels first with a vacuum pump and everything worked great. I think I have almost all the air out. I will pedal bleed when the fronts are done. However, the front brakes won't bleed. I ran the pump for over 3 minutes and nothing is coming out. So I removed the line the goes from the master cylinder to the prop valve and nothing, its dry. There is no fluid coming out the front half of the master. I bench bled and got all the air out, or at least I thought I did. Why is there no fluid flowing from the master to the prop valve?
Intermediate
Joined: Oct 2018
Posts: 36
Likes: 8
I'm doing a frame-off rebuild on my 72 and replaced all the brake lines from master cylinder to calipers. I bench bled and installed a new master cylinder. I started bleeding the lines with the back wheels first with a vacuum pump and everything worked great. I think I have almost all the air out. I will pedal bleed when the fronts are done. However, the front brakes won't bleed. I ran the pump for over 3 minutes and nothing is coming out. So I removed the line the goes from the master cylinder to the prop valve and nothing, its dry. There is no fluid coming out the front half of the master. I bench bled and got all the air out, or at least I thought I did. Why is there no fluid flowing from the master to the prop valve?
mshawn
Thread Starter
Pro
Joined: Jan 2017
Posts: 653
Likes: 51
From: Beaver County PA
That's what I did. I got air bubbles in both reservoirs. I pushed it as far as I could as it got real hard to push, until the air bubbles stopped. Maybe I didn't go far enough. It has the two bleeders on the master cylinder, maybe I'll try the vacuum pump on those bleeders.
Drifting
Joined: Nov 2013
Posts: 1,547
Likes: 282
From: Scottsdale Arizona
The last MC I bought came with instructions not to exceed 3/4” to 1” when pushing the piston in while bench bleeding. I installed it and used my Motive Power bleeder and have a perfect pedal. Manual brake 68.
Racer
Joined: Jan 2008
Posts: 295
Likes: 58
From: Arvada Colorado
I'm doing a frame-off rebuild on my 72 and replaced all the brake lines from master cylinder to calipers. I bench bled and installed a new master cylinder. I started bleeding the lines with the back wheels first with a vacuum pump and everything worked great. I think I have almost all the air out. I will pedal bleed when the fronts are done. However, the front brakes won't bleed. I ran the pump for over 3 minutes and nothing is coming out. So I removed the line the goes from the master cylinder to the prop valve and nothing, its dry. There is no fluid coming out the front half of the master. I bench bled and got all the air out, or at least I thought I did. Why is there no fluid flowing from the master to the prop valve?
Drifting
Joined: Aug 2019
Posts: 1,339
Likes: 225
Try to loosen the MC mounting bolts with the lines still attached, just enough to separate it from the Booster push rod. If the fronts now bleed, you might have to adjust the rod so the MC piston doesn't block the hole.
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Intermediate
Joined: Oct 2018
Posts: 36
Likes: 8
some say to not to push piston all the way in. However I’ve always bleed them this way and it removes all the trapped air. This has always worked for me in the automotive repair industry. But I would follow the direction that came with the new master cylinder just to be sure if you are getting bubbles from both sides of the Mc when you push it , then you have a leak in your bleeding tubes. However you should still have fluid coming out of the port on the front brakes when you push it with the line lose or off . If not master cylinder is bad or block somehow
Le Mans Master
Joined: Jan 2001
Posts: 6,743
Likes: 1,385
Really. Buy or Make a pressure bleeder. I bleed my brakes for fun now(not really, but last time I changed my brake fluid is because I was under there just decided to do it...). I made one from a garden sprayer ages ago because I didn't want to wait for a purchase order (I was gonna get one later) and here I am still using it over a decade later because it worked so well.
BUT meanwhile - to address your forward brake line not flowing I would recommend disconnecting the end at the master cylinder and checking to see if it's obstructed.
BUT meanwhile - to address your forward brake line not flowing I would recommend disconnecting the end at the master cylinder and checking to see if it's obstructed.
Last edited by carriljc; Feb 17, 2020 at 02:26 PM.
Team Owner
Joined: Jul 1999
Posts: 65,492
Likes: 230
From: Orange Park Florida
I bet odds it's that damn prop valve on the frame below the m/cyl.....get rid of it, it's tossed over the block the front brake fluid like it's had a failure.....I ditched it on my '72 many years ago, and just put the rear line to the m/cyl rear output/near the firewall, and the front to a r/l splitter and to the forward m/cyl output, if you have a stock m/cyl, drill one small hole each in the top bulges front and rear brakes, install with gasket per normal.....then take your air hose and pressure the front/rear to force the fluid from under the gasket, through the system....and eventually out the calipers per normal with gravity bleeding....
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
Poor advice. Of all the parts in the brake system: booster, M.C. calipers, pads, lines, hoses, parking brake, booster rod, brake pedal, distribution blocks, the Proportioning Valve has the least tendencies to fail. Its on the vehicle for a reason. It could save your life by activating the warning light that there is a loss of pressure.
Its a safety device that some don't understand its function.
Its a safety device that some don't understand its function.
Le Mans Master
Joined: Jun 2019
Posts: 7,658
Likes: 2,470
From: Indiana
Poor advice. Of all the parts in the brake system: booster, M.C. calipers, pads, lines, hoses, parking brake, booster rod, brake pedal, distribution blocks, the Proportioning Valve has the least tendencies to fail. Its on the vehicle for a reason. It could save your life by activating the warning light that there is a loss of pressure.
Its a safety device that some don't understand its function.
Its a safety device that some don't understand its function.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
That's what I did. I got air bubbles in both reservoirs. I pushed it as far as I could as it got real hard to push, until the air bubbles stopped. Maybe I didn't go far enough. It has the two bleeders on the master cylinder, maybe I'll try the vacuum pump on those bleeders.
DO NOT PUSH PISTON IN MORE THAN 1 3/8 inches. Doing so will destroy the piston seals (rings) and Void The Warranty!
What happens is, if the piston is forced too far in its bore the seal (ring) actually rolls over on itself during the return stroke. This damages the seal.
Take a look at my Profile > Photo Album > Bench Bleeding for info.
Thread Starter
Pro
Joined: Jan 2017
Posts: 653
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From: Beaver County PA
I didn't push it more than an inch in. As the bubbles came out, it started to get hard to push in so that's where I stopped. I usually only get to work on the car on Sundays with my schedule but if I get time this week to check it out, I'll let everyone know what I find out. Thank you for your replies!
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The Distribution Valve (not a proportioning device) doesn't have to FAIL to block off one half of the braking system, THAT IS IT'S PURPOSE!! If your braking system had any kind of leak in the front half of your car, the distribution valve WAS SUPPOSED TO BLOCK OF THAT HALF OF THE SYSTEM.
Once it is 'tripped' to block of the leaking half of the brake system, it must be manually re-centered for it to work correctly....again. There are some 'creative' methods (which I have used with some success) to re-center that valve without manually intervention. But, it must be re-centered for fluid to flow again in that portion of the system.
I'm surprised that some folks giving you 'advice' do not seem to know basic info......
Once it is 'tripped' to block of the leaking half of the brake system, it must be manually re-centered for it to work correctly....again. There are some 'creative' methods (which I have used with some success) to re-center that valve without manually intervention. But, it must be re-centered for fluid to flow again in that portion of the system.
I'm surprised that some folks giving you 'advice' do not seem to know basic info......
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,810
From: Cool Northern Michigan
That's what I did. I got air bubbles in both reservoirs. I pushed it as far as I could as it got real hard to push, until the air bubbles stopped. Maybe I didn't go far enough. It has the two bleeders on the master cylinder, maybe I'll try the vacuum pump on those bleeders.
IDK. I don't have them on a '77.
So, what happens when you just crack open the front brake-line at the MC then press the pedal? Fluid there? If not, the front piston seal might have been damaged.