Front Brakes Hardly Working

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'71 Scamp

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On my '71 Scamp Manual disc on front drums in rear. 71K miles rear brakes seem to be doing all the work and will lock up when applied hard. The rears are not over adjusted and don't need replacement it is all clean back there and appears to be working as it should. I have bled the calipers and they bleed fine but same result.
I was going to rebuild the calipers with seal kits but for them both to act the same is weird I think so I don't want to buy new calipers either. Is it possible the proportioning valve could allow more fluid to the rear than the front? Any thoughts would be helpful.
I'm going to remove a caliper and have someone apply the brake and make sure the pistons move enough and see if its a piston issue.
 

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It is possible that the prop valve could be messed up inside.
It's also possible that both sides pistons are frozen in thier bores.
 
Brake hoses can swell and stop the flow for light braking, then with heavy braking pass fluid, then not release. Symptoms include overly stiff pedal, and not much flow when bleeding.
 
new hoses are the first things you should do. They are pretty cheap, probably need to be replaced anyway and will be one less thing to question. Dont remove calipers and apply brakes. youll blow the piston out and its a hassle rebuilding them unless you use a stop so you can keepo them in the bores. I like the cheap pad spreaders they sell. Also a good bench bleed of your M/C. Then install a plug on the front and see if its nice and firm. Oh yes, there is a chance your prop valve piston is internally rusted and fouled. You can bypass it with a inexpensive adjustable proportioning valve from Summit.
 
What type disc brakes are they Kelsey Hayes 4 piston or the later model single piston? If your get fluid out of the front bleeders than it sounds like the pistons are froze up. Need a little more info. Is this car a recent purchase? Did they work before?
 
Are your pads glazed?

Are you sure there are no pin-hole leaks in the front lines that might cause loss of hydraulic pressure at the caliper?

Another option would be to jack the front up, have someone pump up the brakes and manually rotate the front wheels to see if pressure is being applied.
 
Your problem is most likely bypassing in the master cylinder. I say this because I've seen it so many times.
 
Thanks everyone. So its the Kelsey Hayes 4 pistons Ive had the car for a while and I just thought the brakes sucked but an older mechanic aligned it for me and told me they weren't right. I have a hard pedal, when I bleed it goes to the floor, plenty of fluid coming out. It has enough pressure to stop me from spinning the wheel but when I go down the driveway and apply the brakes hard the back wheels lock up and I skid down the driveway as though the fronts aren't even working. I was gonna use a spacer to see if the pistons moved under pressure, I know they don't travel much but I should see some movement I think. I have purchased seal kits for them but new calipers are $150 a piece, I just don't want to throw money at it.
 
'71 Scamp said:
Please tell me more about this. Thank you.
Click to expand...

He means a bad seal in the master that pushes fluid to the fronts, so the fluid is going past it causing the fronts to not apply but the backs do.

But if you are getting fluid when bleeding, that may not be the case.
(It's possible though)

I think just for the fun of it I would swap the lines at the master and see if the fronts worked.
Don't drive it like that, just test.
 
Was your car originally a front disc car? If not you'll need the correct proportioning valve. Next question, what is the bore diameter of your master. It may be too large for your application. Larger bore requires greater effort. A 16th of an inch can mean the difference between stopping and slowing. On this site , someone has been good enough to post both parts lists and service manuals for our cars. I believe it was AbodyJoe, not 100% on that. Make sure you didn't accidentally get a "power" master where a manual goes. Also make sure your wheel cylinder diameter is correct for your application. Good luck.
 
'71 Scamp said:
Please tell me more about this. Thank you.
Click to expand...

When replacing front linings ( most common service required )
it's common practice to just close the calipers with a clamp, pushing contaminated fluid backward in the system, including the master cylinder. That can damage one half of the master cylinder.
You wouldn't need to jack the car up to prove me right or wrong.
 
RedFish said:
When replacing front linings ( most common service required )
it's common practice to just close the calipers with a clamp, pushing contaminated fluid backward in the system, including the master cylinder. That can damage one half of the master cylinder.
You wouldn't need to jack the car up to prove me right or wrong.
Click to expand...

This causes more harm than most will ever know. You push contaminates back into the Master cylinder bore. You can ruin the rubber cups or clog the discharge port causing the brakes to lock up. Really is a no no to push the pistons back in without the bleeder open.
 
It would probably a good thing to find out if all this just suddenly happened, or it was bought this way.
 
SGBARRACUDA said:
This causes more harm than most will ever know. You push contaminates back into the Master cylinder bore. You can ruin the rubber cups or clog the discharge port causing the brakes to lock up. Really is a no no to push the pistons back in without the bleeder open.
Click to expand...

For all cars or just older ones? The reason I ask, is because I've never heard that on newer cars.
 
bkbond12 said:
For all cars or just older ones? The reason I ask, is because I've never heard that on newer cars.
Click to expand...

There are little details like this one that mfgrs are not going to tell you.
On newer cars... The fluid has changed some but the heat, rubber parts, and metals that contaminate that fluid remains.
The big difference is pushing that fluid back into a antilock brake valve assembly and subsequent cost to replace if and when.
 
RedFish said:
There are little details like this one that mfgrs are not going to tell you.
On newer cars... The fluid has changed some but the heat, rubber parts, and metals that contaminate that fluid remains.
The big difference is pushing that fluid back into a antilock brake valve assembly and subsequent cost to replace if and when.
Click to expand...

Exactly, The brake fluid at the calipers and wheel cylinders are subjected to constant heat cycles because of the friction developed by applying the brakes.
On any car it is a good idea to open the bleeder when your compressing the piston to get rid of this fluid. More so when your dealing with Anti lock brake systems. Why risk it? Open those bleeders!
 
If after you fix the front brake problem and you find the rears continue to lockup you might try smaller (7/8") slave cylinders and arc the shoes at the ends. Mine were grabbing hard, I did both of these mods and they no longer lockup or send the car sideways. I also used EBC "green stuff" pads for the front Kelsey-Hayes setup, they now grab properly and made a huge difference!
 
cudajim said:
If after you fix the front brake problem and you find the rears continue to lockup you might try smaller slave cylinders and arc the shoes at the ends. Mine were grabbing hard, I did both of these mods and they no longer lockup and send the car sideways. I also used EBC "green stuff" pads for the front Kelsey-Hayes setup, they made a huge difference!
Click to expand...

I bought the EBC slotted rotors and pads for my 07 4x4 cummins Dodge. What a big difference! And 37 K miles later they still look to be about 80%.
 
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