Front brakes will not lock up

[Lashcat]

I just completed a 4 wheel disc brake conversion. I closed the proportioning valve and just cracked it open like everyone says to do. I took it out for a test and it stops good but I can't get the front brakes to lock, the back brakes will. I am using a stock master cylinder with the factory residual valves removed. I am thinking I have a bad proportioning valve. Before I did the rear brakes the front brakes would lock.



Any help would be appreciated.

 

[huminajumina]

stock M/c for what year the '71 ? I haven't read up on a 4-wheel disc conversion lately,but you may need a bigger M/C with a 1" piston.

 

[pb4ugo]

Are you running an adjustable porportioning valve? If so you need to adjust it some more, till you get it where you want it. What you describe is like what mine did w/the stk porp valve.

 

[srgould41]

I went through the same issues for almost 3 years. On my '71 I added mini truck outers to get the disc brakes including larger V6 4Runner calipers and Monte Carlo rears. I removed my stock master and booster and installed a 9" GM dual diaphragm booster and a 1" Corvette master. With the proportioning valve dialed all the way I was able to lock the rears in dirt, but not the fronts. I made lots of mods to improve things, but what finally helped was moving to a 7/8" master (from a Toyota MR2). I can now lock front and rear in dirt. Forget pavement, though, unless it's raining. The only way to improve on what I have now is to go hydro-boost.

The recommendation of moving to a larger 1" MC above will only make the pedal firmer and won't help you lock the fronts. I wonder, though, because the parts stores list the '71 as having a 1" master. I thought they had smaller MC's.

 

[bikersmurf]

Is your proportioning valve in the rear circuit? (It should be)

Your front brakes shouldn't lock when going forward in the dry.

You'll need to remove the stock residual valves... Replacement with 2lb ones optional.

 

[srgould41]

I'm afraid I don't completely agree with you.

A properly installed brake system would lock the fronts slightly before the rears (does not pertain to anti-lock brakes). If you can't lock your fronts on pavement then you should not be able to lock your rears either. While it is nice not to have to worry about locking up the tires and skidding to an uncontrolled stop, it's also nice to know you can, and that you can ride that edge for maximum braking when required (God forbid).

My old drum brakes could lock all 4 on dry pavement. I did a 4 wheel disc upgrade, all at once, so I don't know what a disc/drum combo would have felt like.

As Lashcat stated, he could lock the fronts before he installed his rear disks.

Your front brakes shouldn't lock when going forward in the dry.

Lashcat, what you are experiencing is very common. Some people make the upgrade and state they can stop on a dime and lock all 4 if they wanted to. From all the reading I have done over the past 3 years I believe they are in the minority. The only people who seem to have completely resolved the issue went with hydro-boost. I have not gone that for yet so I can't advise further. My brakes are just barely OK at this point so I don't want to dump any more money in them for now.

 

[Lashcat]

Humi, I have a stock master cylinder. Srgo, I will look into that, I had not thought about that. Pb4, I have a adjustable proportioning valve. Bike, it is in the rear circuit. I have rebuilt everything except for 3 lines. I also have to agree with srgo, On every pre anti lock brake vehicle I have own I have been able to lock up all 4 on dry pavement.

I played with the valve this evening. It is still doing the same thing. I did find a line leaking going into the proportioning valve. I will get that repaired and then see what happens.

Thanks for the advice and info guy's. I really appreciate it!