I am installing a new brake system in my car. It is a front disc/rear drum set up with power booster and master cylinder under the floor. Front discs are rebuilt '78 mustang floating calipers. I have 10lb residual valve in the rear line and 2lb in the front, had an adjustable proportion valve in line in the rear but that was discarded for a regular old none adjusting type. the problem is - the pedal goes to the floor every time ( it can be pumped up to near the top though) which sounds like air in the lines, but ... I've replaced every line on the car, the rear wheel cylinders, master cylinder and proportioning valve with no improvement. I can clamp off every rubber hose and the pedal stays near the top of it's throw (which is what I want), unclamp any if them and the pedal goes to the floor. I have been fighting this for the last 3 - 4 months and need help badly - anyone have any suggestions on what to do next ???
Ok, try this procedure. Depress and release brake pedal several times. Then hold pedal depressed with medium pressure (25-35 lbs). If pedal does not FALL AWAY (Go to floor) Hydraulic system is not leaking. If pedal FALLS AWAY-hydraulic system is leaking. Check for external leaking at all wheel cylinders/calipers, if no external leaks, there is an internal leak (Master Cylinder Cups). Replace Master Cylinder.
Next. If your system is power brakes and you converted it to front discs, you need to ensure the MC has the correct piston diameter. Normally this would be 1", but it all depends on your system.
Is your regular old Proportioning valve a PV2 designed for Disc/Drum applications? I would recommend a PV2 combination Proportioning valve that provides correct disc/drum balance as well as the 2 lb check valve for your rear brakes.
I pump the brakes several times and hold it (pedal comes up to a decent level) and it does not fade away, it stays right where it should. the master cylinder is brand new (doesn't mean it could not be defective though) and is the 70/30 split. I do have the PV2 proportioning valve. I bought the system - bracket, booster, master cylinder as a kit from either summit or speedway, don't remember which.
OK you have the PV2 valve. Does it have a long (1") extension on the rear brake line? If so, the 10 Lb residual valve is built in and you don't need another one in the system for the rear brakes. This would be a combination valve. I would check with the outfit you got it from though. The other issue I can think of are a mismatched MC to the calipers (small MC cylinder bore diameter) This may not do what you have for issues, or your not bleeding the system correctly. Is the PV2 below the MC. It needs to be.
When you bleed a split system it is necessary to leave one bleeder screw open in the front when bleeding the rear wheels and vice versa. Always use bleeder screws and jars that the lines go into the fluid so no air gets into the open bleeder line. Bleed your system using a hand vacuum pump so you do not get the MC valve misaligned in the bore. Always start from the furthest point in the system form the MC (Longest line).
I use Master Power Brakes for my systems and they match what you have to what you want or provide a complete system. You can call them for help at 1-888-251-2353. Ensure you have the piston size of your calipers, the master cylinder ID.
Good ideas. My PV2 does not have the extension on the back so I must need the external 10 lb residual valve ....but I do not have it mounted below the mc. So I will do that. I like your idea about the mismatch between the mc and the calipers, that would make some sense.
Good tip about the bleeding too, I have not been doing it that way.
And I will definitely give master power brakes a call, I have been talking to cpp and they have helped but not been able to solve anything.
Thanks again.
Okay, so I have tried everything - moved the pv2 below the mc, bled the brakes again (with a power bleeder)and see no air in line (bled twice just to make sure), checked the rear drums. I bought the mc and calipers from O'reily's and they say they are a match - a 1" bore I think is what they said. It's getting worse!
I still have a feeling it has something to do with the front calipers, I can clamp off the hoses going to them and can get good pedal - release one or the other or both and loose pedal. Seems like it needs more fluid than I can give it on the first pump.
enjenjo - what will this check valve do for me?
In my experience, nothing. I have never installed a 2 psi Wilwood check valve that worked. Without a check valve in front, the seals on the calipers will pull the piston back enough to give you a low pedal. A good 2 psi check valve will hold the pistons in place, and you will have a good pedal. In the first post you saod you have a 2 psi valve in the front brakes, it's not working. That's why I asked if it's a Wilwood.
So you can isolate the problem to the front calipers. When you do that are the rear brakes engaging? I hate to continue to give you additional work on testing your system, but sometimes its a needle in a hay stack to locate the problem and once you do, its like, WOW.. Ok, not sure how you bled the system initially (probably via brake pedal). Sometimes the proportioning valve is not centered and get a low pedal condition on one of the circuits (front or rear). This is how to test the proportioning valve.
1. use a test light (12 volt) clip one side to 12v positive and touch the other side to the electrical connection on the PV. If no light goes on, the valve is operating correctly. Nothing else to do on this. If it lights up, your differential valve in the PV is stuck to front or rear position.
2. Bleed brake system to determine which it is. Fluid flow determines the issue (trickle or squirt).
3. The system side with flow must be opened (bleeder valve) for flow to alter the pressure on opposite side of the differential valve to center the valve.
4. Slowly depress the pedal with steady pressure until light goes out. When light is out close the bleeder.
5. At this point your system is now centered. Bleed the complete system using a vacuum pump at each bleeder starting at the most distant point on the brake system and working around all four wheels to the closest bleeder to the PV.
If all this fails, and you can isolate the poor pedal to the front calipers. Replace one and see if it resolves the issue.
I do all my bleeding alone using a hand operated vacuum pump and plastic bleeding bottle designed to do this. If you do not have this simple device and bottle. GO AND GET ONE. Your fighting yourself and will never get the system working. Bleeding brakes using the pedal as your method will never work.
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