Brake bleeding problems....help!

[Mr Twister]

Hi Everyone
I hope everyone is staying safe in these trying times.
I've been working on my 72 4x4 brakes. I Installed new rear cylinders, new lines to the cross over lines at the front bumper, new proportioning valve & a new master cylinder.
I bench primed the master cylinder, added the proportioning valve and primed that with the master cylinder attached. Connected all the brake lines. I removed the switch from the proportioning valve and screwed in a home made stop to keep the cylinder centered.
The front brakes bled out without any problems. No matter how much fluid I pull through the lines at the back wheels I keep getting bubbles. I've went through the process a few different times now and still cant bleed the back wheels.
I'm at a complete loss and could sure use some help or advice here.
Thank you!
Mr Twister

[Aberdare]

Mr Twister:

If you are still getting bubbles, then you have air either still trapped in the system... or being sucked in somewhere. 1st rule to bleed a hydraulic system, after bench bleeding the master cylinder (and did you bleed it properly?), the next step is to bleed the LONGEST brake line 1st... usually the right rear wheel. then 2nd longest... usually left rear. then right front and left front.

Were your "NEW" wheel cylinders actually NEW?... or rebuilt?... or NOS?

When I bleed brakes, I normally use a bleeder ball... it becomes a one person job, and if you have a leak - well, the system is pressurized so the leak will be clearly visible.

[Mr Twister]

Back again.
All parts are new...not rebuilt.
There are no wet spots anywhere on the brake lines indicating a leak.
My next plan of attack is removing the rear brake line from proportioning valve and connecting an air hose with about 15 lb of pressure and check for a leak in the rear line. Then double check all connections at the master cylinder & proportioning valve.
I'm using a vacuum pump to bleed the brakes.
Does any one have a better course of action?
Thanks Again for your support.
Mr Twister

[Rickysnickers]

What kind of proportioning valve are you using? Is it the type that needs to tool to depress and center the plunger? If so, that could be the problem. Fast forward to about the 3 minute mark.

https://www.youtube.com/watch?v=qPLaPv8DITE

[Mr Twister]

I made a home made plunger and believe its holding the piston in center.
The last attempt I tried to bleed the rear brakes I pulled about 8 oz. Of fluid out of the back passenger's wheel and still had a continuous flow of fluid & bubbles.
So let's assume for a minute that my homemade plug is working what other issue could be the problem....if nothing else cooul cause this issue then I I'll, have to check the plug I made.
Thanks
Mr twister

[Aberdare]

Originally Posted by Mr Twister

I made a home made plunger and believe its holding the piston in center.
The last attempt I tried to bleed the rear brakes I pulled about 8 oz. Of fluid out of the back passenger's wheel and still had a continuous flow of fluid & bubbles.
So let's assume for a minute that my homemade plug is working what other issue could be the problem....if nothing else could cause this issue then I I'll, have to check the plug I made.
Thanks
Mr twister

The Brake Master Cylinder should be in the "Rest Position" then use a bleeder ball on the master cylinder reservoir. It will provide fluid pressure to move the brake master cylinder off the "rest position" and allow fluid to pass, no more than 8-10psi... note; this will be fluid without any air in it, and it will also pressurize the brake lines, hoses, fittings and ultimately the wheel cylinders, as a COMPLETE brake hydraulic system. So, if there is a leak, it will show. The problem with using a vacuum pump is that it will draw fluid and if there is a leak, it will draw air thru that leak... no matter how small it may be, hence the air bubbles you are getting. A Vacuum Pump is really the wrong tool to use for this job. Get yourself a Bleeder Ball, that's what they were made for.

[Mr Twister]

I will agree with you in that I never really liked a vacuum pump to bleed the brakes but it seemed to get me by until now. Thanks for your suggestion to use a ball/pressure bleeder. It makes perfect sense. Although I'm 70 and this thing may sit on my shelf the rest of my life I think it's worth getting one just to win this battle.

Thanks again for your interest in my problem. I'll let you know how I make out and what I find.

One other thing. It sounds like I shouldn't exceed 10 lb. Of pressure to the master cylinder. As for the proportvale I still need to use the plug to keep the center piston from moving....right?
Thanks again and stay safe!
Mr Twister

[Mr Twister]

Brake update.
Successfully pressure bled the brakes via through the master cylinder. No bubbles any more but not a really good flow. I test drove the truck and had great brakes until I got back home, about 2 mile. Then backing around the driveway the peddle fell back to the floor again. I'm really at a loss. Had a slight issue keeping red dash brake light out but I think I was successful.

[Aberdare]

Originally Posted by Mr Twister

Brake update.
Successfully pressure bled the brakes via through the master cylinder. No bubbles any more but not a really good flow. I test drove the truck and had great brakes until I got back home, about 2 mile. Then backing around the driveway the peddle fell back to the floor again. I'm really at a loss. Had a slight issue keeping red dash brake light out but I think I was successful.

Was the reservoir still full? The Red Warning lamp is an indicator, you have either a mechanical fault, or a hydraulic leak somewhere. Fluid leaks tend to be easy to find as they will show up easily... a mechanical fault - not so easy.

[Mr Twister]

While pressure bleeding I checked for leakes.....none. I also used a proportioning valve screw in stop to hold the center valve.
Going to the shop next week to let the pros take a look.
I'll post what the find.
Thanks again for everyone's interest and help with my problem.
Mr Twister