I am having a heck of a time getting the brakes figured out on this truck. I have put on new steel lines, rubber hoses, wheel cylinders, brakes, and master cylinder on this truck. The pedal will stiffen up, but will bleed off if I hold the pedal. I have tried bleeding several times and it doesn't seem to help. The brake light is on and I have tried to get the proportioning valve reset by cracking the lines open with no luck. The vaccum pump seems to be pulling vacuum, but I do not have a vacuum guage to check the exact reading. How do I know if it is just a stuck proportioning valve, booster, or what else do I look for? I am out of ideas.

 

1. If the proportioning valve is off center then you likely would not get very much, if any, flow when bleeding the brakes (to front or rear depending to which end the valve is offset). Pull the wire off the proportioning valve contact and see if the brake warning light goes out. If it doesn't, check the parking brake switch circuit. If the proportioning valve is offset, the you need to loosen the lines on the opposite direction. Sometimes you have to stomp the pedal pretty hard to get the valve to re-center. 2. When you replaced the master cylinder, did you make sure the adjustment on the pushrod between the booster and master cylinder was correct? Too short, the pedal goes too far down and may feel spongy. There should have been a little cardboard gauge included with the new master cylinder (or at least adjustment instructions). 3. Possibly a leaking booster, vacuum line or weak pump. 4. When you replaced the master cylinder did you bench bleed it in a horizontal (level) position? It's virtually impossible to get all the air out with the cylinder mounted because of its angle.

 

I had the same problem prior to replacing the master cylinder. I replaced it hoping to fix the problem to no avail. I have tried and tried to get the proportioning valve reset. Where do I crack the lines at? I have been cracking them at the master cylinder, should I try at the valve itself? I thought if the booster or vaccum pump was bad I would have stiff brakes. How can I check the vacuum pump and booster without just going out and buying parts that do not need replaced. I did bench bleed the master cylinder. Can I take the proportioning vavle of and reset it any way?

 

The easiest way is to open the bleeder ports at the wheel cylinders. I'd try opening the front bleeder port first. You probably only have to open one side (L or R). The "Brake" light should go out when the differential valve piston is centered (assuming no problems in its electrical circuit). You may have to push the pedal much harder than when bleeding to get the valve piston to move. You could probably do it on the bench using compressed air to move the valve piston and an ohmeter to determine centering. The resistance between the switch terminal and the valve body should be a short when off-center and open when centered. Some valves can be manually centered using a piece of stiff wire like a straightened paper clip but it depends on the particular type of valve.

 

I removed the valve and it is shot. The valve is seized bad. I went out and took one off of my old 79 f250. The valve is centered and the light is off, but I did not have anyone to help me bleed them, so I will rebleed the brakes tomorrow and go from there. I guess if this doesnt work I need to start looking at the vacuum pump/booster. How much vacuum should the pump be pulling? Thanks for the help.

 

Was there a happy ending to the scenario? Was the issue solved with another proportioning valve?
I'm having a similar issue.
If you step lightly on the pedal while driving-it will go right down to the floor and feels just like someone opened up a bleeder screw.
If I STOMP on the brakes-I can lock up the tires and stop reeaal quick.
No, my dashboard dummy brake light is not on.
I do know the light works-it comes on when the parking brake is on.
So, did the proportioning valve solve the issue? I've got a spare I can throw on...

 

Your problem sounds like the master cylinder, not the prop valve.....

 

Thank you for the reply. Tomorrow I have theday off from school, so I'll be diving into the brakes. I plan on checking out the rear brakes first to adust them if necessary. After that, I'll do my Master Cylinder because I have a used one in stock. If that doesn't change anything; I'll shoot for the proportioning valve (have a used one in stock). Thank you.

Thank you to all the Veteran's who have served our country! Your sacrafice has allowed us all to maintain our freedom. It's something that is taken for granted daily. Makes me wonder why we only give ONE DAY out of the whole year to thank our Veterans for ther duty, while we are free the entire rest of the year...Thank You for your duties.

 

Just an update-always nice to have members report what solved their issue...it makes people (like me) more apt to use the search feature rather than rattle off the same question time and time again.
I ended up adding brake fluid and bleeding the brakes a few times before I really dove in. I ended up breaking a bleeder screw on the driver front caliper.
I had spare calipers "in stock" with only about 6 months/5,000 miles use so I threw them on. After a few rounds trying to bleed...didn't change anything.
I threw on a spare used master cylinder and that has seemed to fix it.

 

Just so you know for the future the easiest way to check your vacuum booster is to pull the line out of the booster...if the engine struggles to stay going then it is working...hope this helps others out there! Glad that you were able to figure out that problem!