Brake Pressure Problems
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Laughing Gas
Joined: Feb 2003
Posts: 1,242
Likes: 2
From: Bartlett, TN
Brake Pressure Problems
I hope I am not having an unusual problem withm my brakes. I just installed new SS brake lines, a new master cylinder (original from MidFifties), and new shoes and springs and wheel cylinder for 9" rear end. Now every other day I find that my brake lights stay on because there is pressure building up in the line! Do I have air in my lines or a bad check valve or too much brake fluid or something else entirely? When my lights stay on I have almost no pedal travel. After relieving one of the wheel cylinders (the bleeder screw), everything is OK.
Mountain Pass
Joined: Oct 2003
Posts: 138
Likes: 0
From: Near Orlando, fl
Hi there, Just a thought, I recently rebuild my master cyliner. When I when to bleed the brakes, no fluid, however, I had good pedal pressure. The problem was debris in the flex hose to the wheels. How old are the hoses? Also, the pressure sensor for the brake lights, did you you replace it when you rebuilt the master cylinder? Best of luck, Monte
Freshman User
Joined: Dec 2003
Posts: 33
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From: kamloops
It could be your combination valve, it has 3 parts, front valve meters the flow of fluid to the front brakes to prevent the locking up first. Middle is brake warning light detects pressure loss in one of the curcuits causing a piston shiftover putting the brake light indicator on blocking fluid loss.Rear proportional valve prevents rear wheel lockup.
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Laughing Gas
Joined: Feb 2003
Posts: 1,242
Likes: 2
From: Bartlett, TN
I needed to give more information. I used a new, in-th-box master cylinder that came with the truck when I bought it. I don't know how old it was, but it was still in the plastic. I also bought new wheel cylinders for the rear brakes. This is an original single diaphram master cylinder, so it does not have a combination valve.
I did let it sit for a long time between starting to install much of the parts and finishing with new DOT4 fluid. I also used a one man bleeding kit that I was not convinced worked well. I might try to rebleed the lines the old fashioned 2 person way.
I did let it sit for a long time between starting to install much of the parts and finishing with new DOT4 fluid. I also used a one man bleeding kit that I was not convinced worked well. I might try to rebleed the lines the old fashioned 2 person way.
Senior User
Joined: Oct 2003
Posts: 358
Likes: 0
From: Joyce, WA
On my f600 I went thru the same type of problem, only I could only get pressure if I pumped the pedal. I adjusted the rod that goes into the mastercylinder. This firmed up the pedal all right, right to the top and my brake lights were always on. I backed the adjustment of a couple of turns and taa daa. Nice firm pedal, a little travel and the brake lights are only on when I step on the brake pedal. If your truck has an adjustment for the pedal rod into the master cylinder I would adjust that first. Hope this helps!
Laughing Gas
Joined: Jul 2003
Posts: 1,224
Likes: 19
From: scotia. NY 12302
glenn, i had the same pressure problem when i had my '49 on the road (1980's). i had a new master cylinder from napa and tried many options to fix it to no avail. i finally turned it over to an old timer, he changed out the new relief valve with and different one, said it had come with the wrong pressure relief valve, that's the only thing he changed, and it worked fine. good luck, dick r.
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Laughing Gas
Joined: Feb 2003
Posts: 1,242
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From: Bartlett, TN
I'm the oldest oldtimer I know! I might have to look at the pressure valves. There is only one main outlet on my master. Would the valve be there, or maybe in the brass outlet fitting that is mounted to the back? I wonder if there is any specific order to mount the lines to the outlet?
Does everybody grasp at straws like this when nothing makes sense?
Does everybody grasp at straws like this when nothing makes sense?
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Laughing Gas
Joined: Jul 2003
Posts: 1,224
Likes: 19
From: scotia. NY 12302
glenn, my recollection was that the relief valve was inside the master cyl all the way to the back, next to the outlet fitting. shop manual (page 223)shows part "2178 valve". my mastercylinder was new out of the box, but my mechanic friend claimed it had the wrong relief valve in it. i had the same symptoms that you have. i'm no mechanic, hopefully the more knowlegeable folks on the site will help out here. dick r.
Lead Driver
Joined: Apr 2002
Posts: 8,813
Likes: 13
From: Littleton, New Hampshire
Glenn,
It would take a real expert to be able to look at the internal relief valve and be able to determine it was the wrong one. I am under the impression that all single chamber master cylinders had the same relief valve for all drum brakes. I have disassembled several master cylinders and removed the internal relief valves, but never have I replaced one with something else. If you disassemble the master cylinder take carefull note of the order everything comes out to be sure it goes back in the same order. The relief valve is the last thing in there like Dick said, and be sure you don't leave it out when you put it all back together. An all drum system requires a relief valve or you will have excessive pedal travel. It could be that the relief valve is defective as opposed to incorrect...
It would take a real expert to be able to look at the internal relief valve and be able to determine it was the wrong one. I am under the impression that all single chamber master cylinders had the same relief valve for all drum brakes. I have disassembled several master cylinders and removed the internal relief valves, but never have I replaced one with something else. If you disassemble the master cylinder take carefull note of the order everything comes out to be sure it goes back in the same order. The relief valve is the last thing in there like Dick said, and be sure you don't leave it out when you put it all back together. An all drum system requires a relief valve or you will have excessive pedal travel. It could be that the relief valve is defective as opposed to incorrect...