Ok, so I've had this problem for a long time. The car is traditional but the front disc brake setup isn't. ( GM A-Body stuff) It's a disc brake conversion on the front of my Chevy. The problem is that it never fully lets the pressure off of the front calipers. The calipers, pads, master cyl., etc. are all fine and have been replaced and the problem remains. The car is drivable but the drag on the brakes is killing my MPG. Wondering if it may be a proportioning valve problem or something. Anybody else ever had this?
You can start by cracking the bleeder on each wheel and seeing if there is pressure. If that allows the wheel to spin freely start the check list. Pedal free play. Does the pedal return all the way? Spring? Does it have check valves in line or built into the master? Are the pistons sticking in the calipers, or do the pins/slides need to be cleaned and lubed? More info gets better answers. Brake drag that kills mileage would kill pads and rotors way before you noticed that your mileage is bad.
It's not an extreme amount of drag but it's enough that it takes a bit of force to turn them by hand. Check valves are in line. Brake pedal comes up like it should. Not a huge deal I guess but it really gets on my nerves. Chopped51- Never thought of that, I'll check it out.
Just because the brake pedal come all the way up doesent mean its not putting pressure on the M/C.....I had the very same problem and had to shorten the push rod and added a spring between the M/C and pedal to keep pressure off the M/C...solved my problem..BTW it was under floor board setup
Definatly something to look at I also had the 75 corvette disc brake conversion kit from old dog on a 51 chevy . The brake booster pushrod was to be be cut to length , I ended up welding a small extension on the end of it tomake it a little longer . Changes in all different applications. Otherwise you could also look at the rubber ext. Lines going to the calipers they suck shut on the inside lining. Posted using the Full Custom H.A.M.B. App!
After I did front discs on my 50 Plymouth I had the exact same problem. After checking all the stuff posted here I finally found the C clip that holds the piston in the master cylinder had a very slight bend in it. It was holding the piston in about 1/16 inch at the most. I changed the clip, problem solved. Posted using the Full Custom H.A.M.B. App!
I see someone mentioned the rubber lines... Did you use new ones or recycle ? If you re-used them thats a good place to look , they do get old even when they might look good on the outside, time takes a toll.. Swelling shut on the inner hose is not unusual,, fluid returning to M/C is under only slight pressure and cant overcome the restriction . When the pressure on the application of the brakes is much greater and can "over come the restriction" a very common issue.. have a look.
You said you had check valves inline. Did you check to make sure that the front residual valve (check valve) is a 2 lb. valve and not the 10 lb. The 10 lb. valve would keep drag on the rotors. Bob
Try what brokenspoke recommended first, the pushrod should have slight play between the pedal and the master and it is the easiest to repair.
You don't say if you have hanging pedals or not. Check valves are only needed if the master cylinder is under the floor. Is the master cylinder one for disk brakes?
For what it's worth, disc brakes never seem to fully release from the rotors. At least not enough to slip a feeler gauge between them. If you have a true dragging problem, enough to cause you fuel economy problems, I'd expect that you would have toasted your rotors by now.
What he said......gravity takes care of the problem the check valves are put in line to solve with "under the floor" applications.
Me thinks it's pushrod length... you need about 1/8 - 1/4" free play (measured at pedal). Start there, then work your way to the calipers, troubleshooting along the way.
The only thing that makes disk brakes back off is that they use a square O ring it deforms when applied and trys to come back to it's orginal shape when the pressure is released. If enough wear happens it will slip on the piston.
I've had rubber hoses cause calipers to drag, but unlikely, I think, that both would be bad. Pushrod length/pedal free play sounds like a good start.
Going to check into the push rod length thing first, sounds like the most likely culprit given my circumstances. It's the stock pedal setup on a '57 Chevy (Car) with a later style booster and M/C.
If the front brake drag is too abnormal, you would see/smell obvious smoke or lining odor, as has been mentioned. Normal disc lining to rotor clearance is only a few thousands, created by the caliper piston seal retraction. More clearance is normally gained by rotor and bearing run out while the vehicle is in motion. There are two critical push rod issues with a vacuum booster: One is the clearance between the booster push rod and master cylinder primary piston, which is usually around .040", and checked with full vacuum (20" Hg) applied to the booster. With matched OE production parts, this dimension should be OK. The second is making sure the brake pedal can return (move) farther when disconnected from the booster push rod. This ensures the booster fully retracts when the pedal is returned. A separate pedal return spring is also required, as the internal booster spring(s) are not designed to return the pedal. With the master cylinder firewall mounted, no residual valve is needed for disc brakes, but vintage pre-'70s drum brakes do require a residual, and they also can help improve pedal feel with later drum brakes. I would suggest using a stand-a-lone adjustable proportioning valve v.s. any fixed-value "factory" combo valve with custom non-stock systems, IMO.
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