Help me improve my brakes

[RGAZ]

I agree, and the adjustable in the rear line is acceptable but this could cause the brake light to turn on if the difference is too great and plumbed before the dist block. Ideally you would plumb it after the dist block and switch or you can disconnect the light.

From my actual car, the braking was poor and yes the rear locked earlier until I changed the prop valve. He wanted improved brakes, this will get him improved brakes.

 

[slantsixdan]

The calipers with 2.75” pistons are a factory caliper and were used on everything but A-bodies.

The calipers with 2.75" bore are a factory caliper used on '76 A-bodies (and then a bunch of other applications, too: F-bodies, etc).

Any factory disk brake master cylinder should be fine for volume displacement.

Agree.

Further, I have personally run the 2.75" calipers with factory A-body master cylinders that originally had 2.6" calipers and with DoctorDiff's 15/16" master cylinder, volume displacement is not an issue.

Agree.

None of the systems - distribution block or factory proportioning/combination valve, reduce the pressure to the front brakes. So, the disks will get the same amount of pressure with either system, and that is determined by the master cylinder bore. So the disks will function the same.

Agree.

Now, the distribution blocks were for drum/drum cars, and they don't proportion. So the pressure sent to the rear drums may not be what you need with disks up front, because the only front/rear brake bias on the drum cars came from the wheel cylinder and drum size. The factory proportioning/combo valve reduces pressure to the rear drums.

…which is really important; see the rear-brakes link in post #16 of this thread.

 

[72bluNblu]

I agree, and the adjustable in the rear line is acceptable but this could cause the brake light to turn on if the difference is too great and plumbed before the dist block. Ideally you would plumb it after the dist block and switch or you can disconnect the light.

From my actual car, the braking was poor and yes the rear locked earlier until I changed the prop valve. He wanted improved brakes, this will get him improved brakes.

Yeah the only proper way to plumb an adjustable valve is after the distribution block, which is why I said it has to go in the rear line on the other side of the distribution block.

And I never said installing a factory prop valve wouldn’t work, I was just giving additional options that involve less plumbing.

There are also some folks here somewhere just running a distribution block with disk/drums. Personally I don’t think that’s a good idea, but depending on the cars suspension and tire set up I’m not gonna say it’s impossible to have a decent bias for a particular set up doing it that way.

The calipers with 2.75" bore are a factory caliper used on '76 A-bodies (and then a bunch of other applications, too: F-bodies, etc).

Dang it, I forgot about the ‘76 A’s! But yeah, standard on a lot of different Mopars from the era.

 

[RetroDude]

Very interesting following this thread as I have a similar problem as DodgeBoy. The car is a '70 Dart Swinger 340 with factory power front discs. When I got the car, it had NO brakes (pedal to the floor). I installed a new M/C. Now, the pedal is high and hard, but, the pedal pressure required is just nuts. I don't think lock up is even possible. The car brakes smoothly and evenly without issue except for the crazy pedal pressure. I don't think the booster is working, but, I don't think that's the issue. I have another '70 Dart that was a four wheel drum brake car that I converted to Wilwood front discs and used the same M/C that was in the car. That manual brake car stops fine from 120 mph without issue. That kinda makes me think it's not a M/C issue. I'm perplexed!

 

[RGAZ]

I don’t have much more to add. I added an adjustable prop valve in front of the dist block because I don’t use the brake light and got the rears setup nicely. My manual brakes are hard (duh manual) but they can stop the car quick when I stand on them if needed. Not like modern brakes but probably correct for 1973.

 

[Dana67Dart]

The car brakes smoothly and evenly without issue except for the crazy pedal pressure. I don't think the booster is working, but, I don't think that's the issue

If the booster is not working that is the problem.

Manual brakes take considerable leg pressure.

If the MC is smaller (or larger, I don't recall which right now ) than 1" the leg pressure is increased.

I'm assuming you have stock Kelsey Hays front disk and 10x1.75 rear drums.

Power brakes have a different ratio vs manual so without the vacume assist it is typically harder.

Diagnose your booster before tossing parts at it

 

[RetroDude]

If the booster is not working that is the problem.

Manual brakes take considerable leg pressure.

If the MC is smaller (or larger, I don't recall which right now ) than 1" the leg pressure is increased.

I'm assuming you have stock Kelsey Hays front disk and 10x1.75 rear drums.

Power brakes have a different ratio vs manual so without the vacume assist it is typically harder.

Diagnose your booster before tossing parts at it

Thanks for the tip. I've been undecided on what action to take for some time. I will follow your advice and address the booster issue and see what happens.

 

[Dana67Dart]

Easy to test.

Remove the vacume hose and cap it

If the brakes feel the same the booster is not helping.

 

[Bewy]

Good, simple, test^^^^

 

[oldkimmer]

Easy to test.

Remove the vacume hose and cap it

If the brakes feel the same the booster is not helping.

Or press your foot on the pedal and then start it. Does the pedal go down then kick back. If so the booster is good. Kim

 

[RetroDude]

I really appreciate all of the tips. I do think the problem is a bad booster. The next problem is finding a correct replacement. There's about a zillion to choose from and few provide detailed specs. I'm thinking about replacing the MC (again) with one for manual brakes (larger diameter cylinder??) and leaving the dead booster in place. Actually, I would prefer to convert back to manual brakes, but, I just don't know what kind of linkage issues I would run into. I'm 79 and don't fit under the dash like I did when I was 40!!

 

[CFD244]

I really appreciate all of the tips. I do think the problem is a bad booster. The next problem is finding a correct replacement. There's about a zillion to choose from and few provide detailed specs. I'm thinking about replacing the MC (again) with one for manual brakes (larger diameter cylinder??) and leaving the dead booster in place. Actually, I would prefer to convert back to manual brakes, but, I just don't know what kind of linkage issues I would run into. I'm 79 and don't fit under the dash like I did when I was 40!!

Just send your original booster in for a rebuild. There are several reputable shops that do it.

 

[RetroDude]

Can you suggest one?? Thanks in advance.

I did get a quotation for rebuilding my booster from Brake Materials and Parts, Inc. and it made my head hurt!! $345 plus $40 shop fee plus additional parts (if needed) plus shipping both ways. Holy crap!!!

 

[Mojoe9955]

I personally don't care for power brakes. I usually remove the booster and replace the master cylinder with a factory disc/drum one. Usually all that is required is to massage the lines to fit and replace the pushrod the correct length or an adjustable one. I left the original distribution block and on 3 out of 4 cars it worked fine. I did however have to add an adjustable prop valve for the back drums on my 69 RR as when braking hard they would lock if you didn't have time to "pump" them.

 

[RetroDude]

I'm happy to report success!! I installed a replacement booster and, now, the car stops!! I had to replace the push rod linkage between the brake pedal and the booster. I never want to do that, again. I'm 79 and not as bendable as I used to be. I removed the driver's bucket and laid on the car floor on my back working on that linkage for hours. BTW, for those who have never done it, the shoulder bolt and nut are are center and above the steering column - pretty much no see, no touch!!

I want to thank everyone for your tips and support!!

 

[Bewy]

79? You have earned a beer....maybe two...

 

[RetroDude]

"79? You have earned a beer....maybe two..."

I agree!!!