1974 Brakes

[John D.]

I am just finishing a long restoration on a 74. I drove it for the first time and it was difficult to stop. I examined the calipers, rotors and pads and all looked new. I drove it some more and on a major freeway at about 80 mph, I lost most of the brake pedal. I nursed it home and after some consultation, I replaced the master cylinder. I hace the brakes examined and bled properly. Two days later I was on the way to a car show and lost the pedal again. This time I replaced the proportionaing valve and all of the lines to the rear brakes. I again had the brakes bled. The pedal remains, but the car still won't stop in any reasonable distance. I put the car on jacks and spun edach wheel while someone applied the brakes. Each wheel immediately stoped. I am at a loss what to do next. Does anyone have any ideas?

[Harmon C.]

Re: 1974 Brakes

John
Glad to hear the 74 is on the road but you need brakes at 80mph. Reduce the speed till you get them fixed. I was just through this with a 73. Make sure the master cyclinder fits against the booster with no tension when drawing down the bolts. My rod was adjustable and by fixing this and another new M/C I got mine working fine. It took me about three M/C before I got a good one. I had one that was good when removed to put the rebuilt original back on a few years ago and it was no good when put on this car. I don't think the runout on the rotors could cause your brakes to be this bad.

Lyle

[Terry M.]

Re: 1974 Brakes

Don't ignore the rotor run-out -- especially if you get air out of the brakes when you bleed them and it is good when you start but goes bad as you drive. Those are prime symptoms of air entering due to rotor run-out. It is fairly easy to check for rotor run-out.

[Tom R.]

Re: 1974 Brakes

I'm not recommending this but it's worked on my 78 as well as a full drain, drip bleed, rebuilding of M/C and calipers and using O rings. I left the spring outs which eliminates the effect of runout and the runout causing the pistons to pulsate.

[Joe L.]

Re: 1974 Brakes

John and Terry-----

Yes, I agree. You want to be sure that the rotor runout is right before looking elsewhere for this problem's source. This is especially true if the rotors have ever been seperated from the rear spindles. You can easily tell if this has occurred by noting if the rotor-to-spindle rivets have been drilled out and removed. All you need to do the job is a dial indicator and indicator stand.

Also, if the front rotors have ever been removed from the hub (i.e. the front rotor rivets have been drilled out), then you want to check the front rotors, too. The exact same problem often identified as applying to the rear brakes can happen to the fronts, too. So, if the front rotors have had the rotor-to-hub rivets drilled out, you need to check them for runout.

For the fronts, I recommend that the spindle nut be turned down to "0" end play for the checking procedure and returned to proper adjustment afterwards.

For the rears, there is no practical way to remove any end play, but you should not have more than .008 end play when you begin the procedure. Also, I recommend that the half shafts be disconnected and brake pads removed before performing the rotor runout checking.