New Brake System Losing Pressure

[ChevyBrad]

I installed a new 1.032" Bore Strange engineering dual master cylinder, rear proportioning valve and new lines over to the right side frame from the master cylinder. I bled the master cylinder, tried bleeding the brakes by pumping and cracking the bleeders. Removed all the air I could and if you pump the pedal 3 times it holds real hard. But if you let it sit for a couple seconds it goes right to the floor. I was told I needed residual pressure valves and that should fix it. Installed and bled and same deal. My linkage travels from fully extended to bottoming out in the master cylinder so that's not the problem. I have since installed a new rear axle in the mean time and have the shoes adjusted up perfect. I switched out my master cylinder for my dads which is the same thing but with a 1.125" bore and still nothing. I borrowed a blue point vacuum bleeder from a Co-worker and now just barely have a pedal but not all the time... I am almost out of ideas... the original master cylinder worked great and I had not changed anything when I first installed the new master cylinder. Sorry for the long write up but, I needed all the detail. Any suggestions? Thanks.

[Rude Dude]

Good info here

http://www.classicperform.com/TechBo...eshoot.htm#why

[mr48chev]

The link that Rude Dude provided gives most of the normal issues.

1. What is the brake combination you have, Drum/drum, drum/disk or disk/disk?

If you put an aftermarket tricky slicky master cylinder on it with drum/drum you may not have the residual valves to maintain the pressure you need to keep in the system.
Stock drum brake master cylinders usually come with a residual (check) valve under the brass seat were the brake line goes in the cylinder. Disk brake cylinders don't have that valve.

If you don't have the residual valve maintaining pressure on the cups in the wheel cylinders you may have air being drawn in past the cups when it sits. The other thing to look at is that you may have a fitting in the line that isn't seated right or isn't sealing right letting a bit of fluid to leak out or letting a bit of air to get in.. That happened on one of my OT rigs that I swapped brake lines on and didn't get a flare right and it leaked.

[OrrieG]

Make sure your calipers are mounted correctly, they will work on either side but the bleeder has to be on the top. If it is mounted on the bottom you cannot get the air bubble out. I know from experience, and it acts like you describe, good pedal then nothing when you release it and the air bubble decompresses.

[G&R's57GMC]

Originally Posted by ChevyBrad

I installed a new 1.032" Bore Strange engineering dual master cylinder, rear proportioning valve and new lines over to the right side frame from the master cylinder. I bled the master cylinder,

Are any of the lines higher then the master cylinder ? That would cause a air pocket .

[ChevyBrad]

Oh right sorry, it is a 3800 with stock drum brakes up front and brand new brakes in the rear on a dana 60 I just installed. It is a disk brake intended master cylinder because I was planning on eventually going to disks up front but not yet. I do have 10# residual check valves that I plumbed in on the opposite side of the frame from the master cylinder... Do you think they need to be closer? I was trying to eye ball be heights of the lines and master cylinder and I will have to recheck to make sure they are lower. I did also have a fluid leak at the residual valve fittings but I have stopped that and re-bled so I'm not sure. Thanks for all of your input so far.

[ChevyBrad]

Also to note my master cylinder is mounted at the same height as the stock one. I also noticed that when I crack my left front bleeder screw open after bleeding it I do not get any fluid running out.

[ChevyBrad]

Well I actually measured everything and my master cylinder reservoir level is indeed the highest point by a couple of inches. I decided to put the old M/C back in he system that came on the truck and was working perfect, bench bled M/C tried bleeding again for a while and nothing, the pedal actually feels worse than the new master cylinder. I came up with an idea of how to make my own pressure bleeder from the top of the M/C out and with 20 psi I was able to get a lot of air out of the rear. I still do not have a pedal though. Did not have time to try the front yet but hopefully more air will escape.

[solidaxel]

Originally Posted by ChevyBrad

Also to note my master cylinder is mounted at the same height as the stock one. I also noticed that when I crack my left front bleeder screw open after bleeding it I do not get any fluid running out.

Follow that line back to the master/c and at every joint open the system and see if you have fluid there.............keep going you will find the problem!

[parkwood]

Originally Posted by ChevyBrad

Oh right sorry, it is a 3800 with stock drum brakes up front and brand new brakes in the rear on a dana 60 I just installed. It is a disk brake intended master cylinder because I was planning on eventually going to disks up front but not yet. I do have 10# residual check valves that I plumbed in on the opposite side of the frame from the master cylinder... Do you think they need to be closer? I was trying to eye ball be heights of the lines and master cylinder and I will have to recheck to make sure they are lower. I did also have a fluid leak at the residual valve fittings but I have stopped that and re-bled so I'm not sure. Thanks for all of your input so far.

the residual valves need to be as close to the m/c as you can get them.

[OrrieG]

Did I read right that you are using a drum/disc MC on a drum/drum system? If that is the case you are not getting enough fluid to the front wheel cylinders. The disc lines are much smaller because they use significantly less fluid to push the puck out. Even a full push with the pedal will not push enough fluid thru the line to fully engage the front wheel cylinders.

[ChevyBrad]

Well here is my update on progress today. I rerouted lines so that my residual valves are 8 inched from the M/C. To ensure I got all the air out of the system I disconnected the lines from the wheel cylinders and blew out all the fluid and started from scratch. Sort of the same results happened but I also got a plug from the store and determined that my rear system is working fine and hes real hard pedal but the front when the pedal is at full rest loses pressure again with the rear plugged off. There was A LOT of back flow, about 6 inches of fluid shooting straight up from the reservoir in the front only and nothing from the rear so I ordered another residual valve thinking the old one was shot. After installing with both lines hooked up I had a great pedal absolutely perfect, tested it out for about 5 minutes and stayed perfect until all the sudden the pedal went to the floor when I pushed it down again and its back to its old ways... I see what you are saying OrrieG but I don't understand why I had a good pedal for a couple minutes and then all the sudden nothing...

[G&R's57GMC]

I'm beginning to think its a bad M/C

[parkwood]

I'm thinking like G&R that you could have a bad M/C and the fluid is bleeding back after pressure builds up... can you rebuild the m/c? but better off just getting a new one... somewhere something isn't holding pressure and sounds like the m/c with all the things you've tried..

[Daze57]

had the same problem a long time ago with another truck - m/c was faulty

[OrrieG]

If you have a good pedal and after you keep pushing it drops to the floor I'd go with new MC too. If the truck has sat for any time it happens. I have replaced mine twice during this build because I don't jab the pedal enough on a regular basis to keep the seals tight. #3 is dripping a little but I am going to wait and see if it heals itself when I start driving it.

[ChevyBrad]

Yeah you guys may be right because when I tried my dads M/C in the system I think I still had a lot of air trapped in the system so maybe that's why his didn't work either. Its definitely not from sitting too long because I bought the truck in February and the M/C probably within the month but who knows.

[ChevyBrad]

http://www.ecklerstrucks.com/chevy-t...1947-1966.html

What do you guys think about this? It is listed for my application but I called on it and it only has a 1 inch bore and I am pretty sure factory was a 1.125?

[ChevyBrad]

Now it looks like some of the M/C's that they're selling for the 1 ton are 1.25? I don't know, does anyone have a recommendation for a nice manual brakes dual master cylinder for my application?

[OrrieG]

Someone just posted this link in another brake problem thread. Might help you. http://www.classicperform.com/TechBo...t.htm#testprop

[ChevyBrad]

Well I bought this from Napa today http://www.napaonline.com/Catalog/Ca...974_0358744493 because of what was said on this thread- http://www.stovebolt.com/ubbthreads/...&Number=899528. I haven't gotten a chance to try it out yet but I hope it works.

[ChevyBrad]

Well I installed the new M/C on the truck today with new lines all 1/4 inch like what came on the truck when I bough it and the brakes worked and bled the M/C and bled the rear brakes and the pedal now travels about 2 inches and feels like it hit a brick wall. I went to bleed the front brakes and can't even get air pushed out of them so I unhooked the front brake line and the pedal is still super hard and won't even push fluid out of the front port! The M/C has completely separate front to back systems and has bleeder screws above the ports. with the rear bleeder screw open the pedal will go to the floor with a solid stream of fluid which, is normal, but nothing comes out of the front even tried playing with the fronts bleeder screw and nothing. But Fluid was coming out of the front when I bled the M/C... I think the M/C is junk? I have definitely never seen this...

[ChevyBrad]

Well the new M/C fixed it. I was able to bleed front and back and drive truck around yesterday. Now I just have to fix all the leaking fittings from taking everything apart so many times. Thanks for all your help!