gaveasky Posted September 29, 2023 Share Posted September 29, 2023 Hey guys, since last posting here I have been busy working a lot and fixing the truck in my free time. It's been going ok, but an intermittent problem has just become permanent and I am totally stumped. Once in a while, after starting up and getting down the street, in the first couple of times I applied the brakes I would get nothing and then sudden full rear brakes causing the rear wheels to lock up. I would then drive around the block once or just let the truck idle a few mins and the brakes would be back in business. Yesterday I purchased a couch in town and got in the truck to pick it up, and the brakes are now out of order, when I step on the pedal I get nothing for a few inches and suddenly full lockup in the rear. New master cylinder and booster a couple of months ago. I am hoping the intermittent nature of the problem (before it became permanent) can be a clue, also the fact that both wheels lock up confuses me, but I am by no means a mechanic or expert, I have just been doing what I can on the truck with the help of youtube and forums. I ordered new rear drum hardware kits and wheel cylinders (current ones are leaking a bit for a while) but again, the fact that its both brakes leads me to find it improbable that both cylinders fail at the same time? Here she is: Any ideas/knowledge greatly appreciated. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted September 29, 2023 Share Posted September 29, 2023 Do you have any front brake function??? The fronts should be doing most of the stopping in any case. First off, if you've recently replaced the booster and master cylinder make sure both reservoirs are still full. Two things that could effect the rear only might be the brake combination (proportioning) valve below the *master. This will block off one circuit of the brakes and set the brake failure light on the dash. The other is if you have a 250/350 truck there should be a ride height sensing valve between the frame and rear axle. This is supposed to determine how much weight is on the rear axle and apply more force when loaded. Link to comment Share on other sites More sharing options...
gaveasky Posted September 30, 2023 Author Share Posted September 30, 2023 Do you have any front brake function??? The fronts should be doing most of the stopping in any case. First off, if you've recently replaced the booster and master cylinder make sure both reservoirs are still full. Two things that could effect the rear only might be the brake combination (proportioning) valve below the *master. This will block off one circuit of the brakes and set the brake failure light on the dash. The other is if you have a 250/350 truck there should be a ride height sensing valve between the frame and rear axle. This is supposed to determine how much weight is on the rear axle and apply more force when loaded. I'm gonna have to test the front brake function, I am tending not to hold the brakes down very long as soon as they lock up (dont want to leave too many marks in my neighborhood) - it seems as though the rear locks up before the front does anything though. If I REALLY lightly press on the pedals there is about 0.5 inch of travel where I can stop the truck without skidding but it is extremely sensitive. The reservoirs are both full, even though the wheel cylinders have a tiny leak they have been full for 2 months. No brake fail light on the dash though, and I know it works because it was always on before I swapped the MC. Did not know about this valve, going to look into it. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted September 30, 2023 Share Posted September 30, 2023 I'm gonna have to test the front brake function, I am tending not to hold the brakes down very long as soon as they lock up (dont want to leave too many marks in my neighborhood) - it seems as though the rear locks up before the front does anything though. If I REALLY lightly press on the pedals there is about 0.5 inch of travel where I can stop the truck without skidding but it is extremely sensitive. The reservoirs are both full, even though the wheel cylinders have a tiny leak they have been full for 2 months. No brake fail light on the dash though, and I know it works because it was always on before I swapped the MC. Did not know about this valve, going to look into it. Bill Vose has explained this load sensing valve attached to the rear axle in detail. I'm going to notify him of this thread. In the meanwhile you might want to look at it and make sure the valve isn't bent and still attached as it should be. My truck doesn't have one and I'm pretty certain that if the unobtainable mechanism is broken you could replace the rear flex hose from the crossmember to the axle splitter/vent block with one like like mine. Link to comment Share on other sites More sharing options...
85lebaront2 Posted September 30, 2023 Share Posted September 30, 2023 Bill Vose has explained this load sensing valve attached to the rear axle in detail. I'm going to notify him of this thread. In the meanwhile you might want to look at it and make sure the valve isn't bent and still attached as it should be. My truck doesn't have one and I'm pretty certain that if the unobtainable mechanism is broken you could replace the rear flex hose from the crossmember to the axle splitter/vent block with one like like mine. I guess I should start with a picture of the valve. Here is a view from above with the bed removed: And one from the front (axle was out as I was changing it): The valve reduces the pressure to the rear brakes under light loads, as the load is increased and the frame comes down closer to the axle the pressure is increased to compensate. There is another brake valve up front under the master cylinder, it also regulates the brake pressure to the rear an addition to providing a failure warning. Link to comment Share on other sites More sharing options...
gaveasky Posted October 1, 2023 Author Share Posted October 1, 2023 I guess I should start with a picture of the valve. Here is a view from above with the bed removed: And one from the front (axle was out as I was changing it): The valve reduces the pressure to the rear brakes under light loads, as the load is increased and the frame comes down closer to the axle the pressure is increased to compensate. There is another brake valve up front under the master cylinder, it also regulates the brake pressure to the rear an addition to providing a failure warning. That is one hell of a setup they got there, I just got under my truck and there are definitely two hoses leading aft from the back of the axle and (seemingly) into the frame. I wasn't able to see that scissor mechanism but I am guessing if the hoses are there, it must be hidden under my rear tank, or I didn't look well enough. What's the advice moving forward, any way to check the valve or just delete it seeing as I will never be able to find a new one? God bless you guys, I have a bunch of parts coming in soon so I will plan out what to do. When I drop the rear tank I will be easily able to get in there and check that sensing mechanism out. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted October 1, 2023 Share Posted October 1, 2023 That is one hell of a setup they got there, I just got under my truck and there are definitely two hoses leading aft from the back of the axle and (seemingly) into the frame. I wasn't able to see that scissor mechanism but I am guessing if the hoses are there, it must be hidden under my rear tank, or I didn't look well enough. What's the advice moving forward, any way to check the valve or just delete it seeing as I will never be able to find a new one? God bless you guys, I have a bunch of parts coming in soon so I will plan out what to do. When I drop the rear tank I will be easily able to get in there and check that sensing mechanism out. You should have the brake hose coming down to the splitter block, and then the axle breather hose (fits over the retaining bolt) clipped to the top of the frame rail. I can't help with diagnosing the valve but if the hose fitting on top is the same as 150's and my later truck I could give you a readily available part number that would eliminate the valve. Perhaps someone has already done away with the load valve and that's why the rears grab so hard when the springs hold the bed high? Maybe some pics would help us see what's going on? Link to comment Share on other sites More sharing options...
gaveasky Posted October 1, 2023 Author Share Posted October 1, 2023 You should have the brake hose coming down to the splitter block, and then the axle breather hose (fits over the retaining bolt) clipped to the top of the frame rail. I can't help with diagnosing the valve but if the hose fitting on top is the same as 150's and my later truck I could give you a readily available part number that would eliminate the valve. Perhaps someone has already done away with the load valve and that's why the rears grab so hard when the springs hold the bed high? Maybe some pics would help us see what's going on? I will definitely get some pictures, the truck is just at home and I *just left the house to work* away from home until Wednesday. Im going to come back then with as much info as possible. Its possible the PO did away with it, but the truck was braking just fine both loaded and unloaded until a couple of nights ago... Sorry for not having pics right away, and thank you for your insight. Link to comment Share on other sites More sharing options...
ARM Ashby Posted October 1, 2023 Share Posted October 1, 2023 I will definitely get some pictures, the truck is just at home and I *just left the house to work* away from home until Wednesday. Im going to come back then with as much info as possible. Its possible the PO did away with it, but the truck was braking just fine both loaded and unloaded until a couple of nights ago... Sorry for not having pics right away, and thank you for your insight. Had same problem on 86f350. It was praporting valve. Sorry for bad spelling. Got one from Carolina Classics. Link to comment Share on other sites More sharing options...
ArdWrknTrk Posted October 1, 2023 Share Posted October 1, 2023 Had same problem on 86f350. It was praporting valve. Sorry for bad spelling. Got one from Carolina Classics. Fortunately those are still available. Do you happen to have the part number? Link to comment Share on other sites More sharing options...
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