I just completed building a little hot rod with four wheel disc (kit-Chev) brakes. All new parts through-out. With rear wheels on stands off ground, pressing brake pedal does not stop wheels when in drive gear at idle. Pumping pedal does not slow wheels (posi-traction) to a stop. Have bled system multiple times, master cyl. full. I also have a adjustable prop. valve but don't know which way to adjust it..in or out. Have not done a run out for fear of not stopping and wrecking the car. HELP PLEASE. I'm a beginner!
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What is the make/model/style (general information for) the proportioning valve? I'm thinking it just needs adjusted to solve your issue. I don't think it has anything to do with your front wheels not turning or it thinking the front wheels are in a locked state.– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Mar 24, 2016 at 11:16
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3Pics of the hot rod for, eh, science... ;-)– race feverCommented Mar 24, 2016 at 12:38
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Is the pedal solid? Is the master cylinder designed for rear discs? Calipers need more fluid than wheel cylinders for drums. Were the lines supplied? If not, are you sure they are the correct size? Larger lines needs more fluid. Not sure how much of a difference that would make.– rpmerfCommented Mar 24, 2016 at 13:17
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I'd just adjust the proportioning valve until your rear brakes started to work. Then you can take it out and see if more/less bias is needed. If you've got huge rear tires and small front ones then you'll probably want to bias it more towards the rear than you would in a normal car. OTOH if you've got normal-sized wheels on the front then go ahead and keep most of the bias up there.– TMNCommented Mar 24, 2016 at 16:37
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Hey, Guys. Thanks for all of your feedback. Really appreciate it. I'm going to do all that you have said over the next few days and report back. Prop Valve (basic) is from Speedway motors. Disc brake kits and lines also sized by Speedway (chevy). Also correct residual valves included. Master Cyl. is dual chamber Ford...commonly used in rodding.– RobCommented Mar 26, 2016 at 13:28
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2 Answers
I assume you have a brake pedal that does not go together the floor. With rear wheels on stands the proportioning valve will think the car is in a hard stop situation and not allow the rear wheels to give full braking. Try supporting the same ear at the axle so the wheels are up under the car as they would be while on the he ground. Then the proportioning valve will allow the rear wheels to get full pressure to stop the wheels. As for the adjustment on the pro portioning valve, when the wheels are in the normal position (supported at the axles), the rear wheels should apply brakes to the rear wheels to stop them.
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4I'm not sure your explanation of a proportioning valve is correct. I could see it doing something like that if ABS was involved, but this is just a "stupid" proportioning valve, meaning it isn't thinking about what it's doing, it is just providing a brake bias front/rear.– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Mar 24, 2016 at 11:14
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@Paulster2 - I was thinking about a proportioning valve that attaches to the body and linked to the axle and changes pressure when the nose of the car dives and the rear rises. This is used mostly in trucks. A proportioning valve generally changes pressures in hard braking situations. The bias is determined more by the diameter of the master cylinder bore and the size of the caliper piston sizes, front and rear, front being larger, rear smaller.– X-tech2Commented Mar 26, 2016 at 3:01
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Even so, how would the back end of the vehicle being up in the air affect the brake bias? How would this be any different than the car going down hill? The type you are talking about is affected by inertia, not by what angle the car is at. I'm not beating you up about it, I just don't think you are on the right track, is all. I think the problem lies in the adjustment of the proportioning valve. OP hasn't updated the question with any more information, either. And no, it was not I who downvoted you.– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Mar 26, 2016 at 3:12
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@rob - I assume if you open a rear bleeder valve you are getting fluid when you push the pedal. If not, open one bleeder valve, Have someone push the pedal til it bottoms out, close the valve. Keep doing that until you have fluid. Do the same on the other wheel. Bleed a couple of times like normal and then check if the wheels stop. Sometimes air gets trapped and will move if you bleed the system this way. Also, are the pads installed correctly? If they are caught somewhere you will have a hard pedal but the pads won't hit the rotor, so the wheels won't stop. Good luck. Let us know how it goes.– X-tech2Commented Mar 26, 2016 at 3:17
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When a car goes down a hill, the front and rear of the vehicle are both the same distance from the ground. When you brake hard the rear of the vehicle lifts up in the rear, and dives in the front. The rear axle remains the same distance from the road, but the body rises while the front gets closer to the road. A prop valve bolted to the body and linked to the axle changes the pressure to the rear wheels to keep the rear from locking up on hard braking. If it had that type of valve and the rear was supported at the body it would keep pressure low "thinking" it was in a hard braking situation.– X-tech2Commented Mar 26, 2016 at 3:26
had a hot rod builder tell me not to use a prop valve with 4 wheel disc so I didn't no problem with the rears locking up
