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Hard pedal - weak brakes
https://slantsix.org/forum/viewtopic.php?t=48169		 Page 1 of 1
Author:  ESP47 [ Mon Mar 12, 2012 10:15 am ]
Post subject:  Hard pedal - weak brakes
I'm at my wits end with these brakes. It's a 67 Valiant with 9" manual drums all the way around. I've swapped the master cylinder for a brand new one with new plunger that Dan recommended. I've bled the brakes all the way and blown out all the lines completely and put in new brake fluid. They bleed just fine, the only problem is when I'm all done, the pedal only pushes down a few inches and it's rock hard. The car's stopping power is probably about 33% worse than my Duster with the same 9" manual drums.

It has new shoes and brake hardware. The hubs are fine and I've replaced two of the wheel cylinders. Nothing seems to help the problem at all. I've already asked on here before but I'm getting to the point where I'm going to have to take it into a shop. Do you guys have any idea what else could be causing this? I've done a ton of research and people always seem to say it's the bore size of the MC but I can't see how that would be a problem with a brand new stock MC that's the original part for the car. I've also swapped the drums off my Duster and that didn't help at all either.
Author:  SlantSixDan [ Mon Mar 12, 2012 10:34 am ]
Post subject: 
1. Flex lines. What is their condition? Have they been replaced?
2. Which two wheel cylinders did you replace?
3. What brake shoes did you use, and how did you bed them in?
Author:  Dart270 [ Mon Mar 12, 2012 12:04 pm ]
Post subject: 
Could be a change in shoe lining material... This can make a big difference.

Lou
Author:  wjajr [ Mon Mar 12, 2012 1:05 pm ]
Post subject: 
Stuck wheel cylinders, and collapsed flexible lines can cause a hard peddle.

If the front wheel cylinders are stuck, making front brakes mostly inoperable, you will have hard peddle, and long stopping distances.
Author:  ESP47 [ Tue Mar 13, 2012 10:19 am ]
Post subject: 
The lines are all original. I can see nothing wrong with them and I don't think they are obstructed in any way since they bleed perfectly fine. I believe I changed the rear cylinders due to one leaking and one just looking beat up. The shoes are Raybestos.

wjajr, I like your idea of the fronts possibly being stuck. I'll have to test that out the next time I can get out there. Just because they bleed fine doesn't mean the pistons are pushing the shoes out correctly.
Author:  SlantSixDan [ Tue Mar 13, 2012 10:40 am ]
Post subject: 
Quote:
The lines are all original.
Get new flex lines on that car now, purely as a matter of safety. It also stands a good chance of fixing the problem.
Quote:
I can see nothing wrong with them
By the time they are visibly degraded, they are at least one traffic death just waiting to happen.
Quote:
I believe I changed the rear cylinders due to one leaking and one just looking beat up.
And the fronts...?
Quote:
The shoes are Raybestos.
Which ones? And how did you seat them in?
Author:  ESP47 [ Wed Mar 14, 2012 11:05 am ]
Post subject: 
I haven't changed the front cylinders yet. They looked pretty new when I got the car. I guess that's one of the next steps I need to take.

I can't remember exactly what Raybestos shoes they were. They were just the ones Napa pulled off their computer for the car. I'm not sure what you mean by how I sat them in. I hooked them in with the pins and attached the starwheel and springs. I made sure they were aligned with the wheel cylinders and then I put the drum on and spun it and adjusted the starwheel until it felt right.
Author:  SlantSixDan [ Wed Mar 14, 2012 12:31 pm ]
Post subject: 
Did you turn the drums? Whether you did or not, the curvature of new shoes usually does not match the curvature of the drums, so the contact area between shoe and drum is very small until you've used the brakes enough to "seat in" or "bed in" the shoes so their curvature matches that of the drums. Until that happens, you have a hard pedal and minimal braking action. It is easy to glaze the shoes during this bed-in process, spoiling their effectiveness -- if this has occurred, sand off the glazed patch. If the drums were not turned and properly surfaced (or at least scuffed with coarse sandpaper to break their glaze) prior to shoe replacement, that will greatly aggravate and prolong the new-shoes brake ineffectiveness. A long time ago it was standard to "cam grind" the new shoes so their curvature would match the drums they were being installed inside, but then we figured out that brake dust destroys your lungs, and nobody cam-grinds any more.

(But really, please do put in new flex lines. All three; two at front and one at rear.)
Author:  65Dodge100 [ Wed Mar 14, 2012 1:03 pm ]
Post subject: 
I have the same problem with my truck. The peddle is hard as a rock but it stops fine. I've replaced or rebuilt all the wheel cylinders and replaced the master cylinder. I really shouldn't be griping about it but it just doesn't feel right. Can the flex hoses make that difference or is it just a safety factor?
Author:  WagonsRcool [ Wed Mar 14, 2012 2:02 pm ]
Post subject: 
Beyond having the shoes properly fit the drums (the shoe linings should be very slightly smaller radius than the drums)- any time you replace brake friction- shoes or pads- you need to "break-in" the brakes. This consists of 15-20 gentle stops from 30-0 mph, with 30-60 seconds between stops. After that it's a good idea to continue "gentle" use for the next couple hundred miles.
Like Dan said, new shoes (or pads) will work poorly until they're seated/ broken in, & will be glazed if you abuse them before that.
Author:  SlantSixDan [ Wed Mar 14, 2012 5:09 pm ]
Post subject: 
Another technique for shortening the bed-in time is to bake the new shoes (or disc pads) in an old toaster oven at 350° for a couple of hours. It'll stink, so do it outside. New pads/shoes outgas very heavily when heated, and the gas tends to cushion the pad/disc or shoe/drum contact, which is exactly what you don't want. Cook the gas out of the pad/shoe before installation and things seat in much more quickly.
Author:  ntsqd [ Mon Aug 27, 2012 5:29 pm ]
Post subject: 
This is very late, BUT what was the old m/c bore size and what is the new m/c bore size? Since it sounds like this was the only thing that changed, it is likely the culprit. Changing the bore of the m/c can have huge effects on pedal effort vs. actual stopping power. The m/c's bore size is part of the total system's leverage ratio and altering it changes the total system leverage, sometimes dramatically.
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