Brake upgrades: power brakes, master cylinders, and avoiding lockup
Valiant.org/Allpar, LLC cannot take any responsibility for the outcomes of these processes. Work at your own risk.
Avoiding wheel lock (lockup) when you hit the brakes
Our 1974 Plymouth Valiant has manual disc front brakes and, along with every other Valiant, drums in back. This is in many ways the best setup, since it incorporates the more effective discs without the loss of control that comes too often with the power brakes. However, the rear tires lock up far too quickly when the brakes are applied in emergency situations. Two approaches were suggested, one by Daniel Stern, and one by Scott Sigethy.
Daniel Stern wrote that the key was to replace the rear wheel cylinders with smaller bore versions. This reduces braking power to the rear wheels, balancing the system more effectively so that they are not so prone to lockup. Daniel suggested the 3/4 inch rear wheel cylinders with our setup — parts: NAPA 37863, Raybestos WC-37863, Bendix 34076, Wagner WC123412, Centric 134.67015, or Chrysler 4423 852. (He also suggested that on cars with the heavy 8 3/4” axle, using a 13/16” unit. On his own 1973 with the same 7 1/4” axle as our 1974, the 13/16” wheel cylinder reduced lockup but did not eliminate it.)
Reducing pedal effort
Scott suggested changing the master cylinder on manual-brake cars to reduce pedal effort. He wrote:
The smaller bore master cylinder has a 7/8" bore, and it comes with cars equipped for power brakes. Manual brake cars have a 1 1/8" bore master cylinder. Basically you're applying the same mount of force, from your foot, across a smaller area in the master cylinder. This has the effect of putting more force on the pistons in the brake calipers. In my experience it was not enough force to lock up the rear wheels.
The other thing you could do would be to get larger bore calipers. which were only available on the Dart in 1976. I don't remember the size difference between 1975 and previous, and the 1976 models. But essentially you get the same effect.
It actually escapes me as to why Chrysler did not equip the cars like this from the beginning — why not put the 7/8" bore master cylinder on the manual brake cars, thereby reducing pedal effort, and put the 1 1/8" bore master cylinder on the power brake cars, thereby reducing rear wheel lockup? Maybe it was a way to get customers to buy power brakes... I don't know. Or someone at the top was making decisions who was too far removed from the final product.
As I was telling you, the one problem you may run into is getting the little plastic grommet that goes over the end of the pushrod, where it goes into the back of the master cylinder bore. Power brake cars won't come with that. But manual brake cars will, because you wind up damaging it when you remove it from the bore. However, you need it because if you don't use it, the pushrod will fall out the back of the master cylinder... which will guarantee an accident because you won't have any brakes!
Neither Mr. Bowen nor valiant.org/Allpar, LLC take any responsibility for the outcome of this process. Work and drive at your own risk.
It is no big deal. If you have worked on brakes before and can take your time and be contentious, here are the steps and parts required:
Get a '67 up all drum M/C [master cylinder], and use your pushrod with it. To convert to power, get the booster and M/C from a car in the junkyard. Make sure the booster has the same bolt pattern as your old master cylinder and that the pushrod for the booster is about the same length as your old pushrod. Look for another A-body around the same year as yours is. Power brakes were an option on some years, but I don't know when they started.
You will have to flare your own lines because the tube nuts needed for the master cylinder are an odd size and you will have trouble getting pre-flared lines with those nuts on them (be sure and double-flare)
Both lines are about 20" long, and need to follow the contour of the original single line. Both will be 3/16 instead of 1/4
The tube nut on the far end of the front brake line needs to have 7/16 inch threads but a 3/16 bore to match up with the old 4-way brake union that will be used only for the front brakes in this conversion.
Get a 3/16 flare plug(3/8 threads), and disconnect the rear brake line from the 4-way union down near the left front subframe stub. Put the plug where the rear line went to, and buy a union to join the male end of the old rear brake line to the male end of the new line that you made to go from the front fitting(for the rear brakes) on the new master cylinder. You will have to bend the old rear brake line up to match the angle of the new line.
Join those two, and disconnect the old 1/4 inch line from the top of the main union that the rear and front lines ran to. Attach the new line that you made for the rear port on the new master cylinder to the place where the old 1/4 line went to. This is the line that needs to have the 7/16 thread, 3/16 bore tube nut on the end.
Bleed and check everything for leaks and hit the road!
Here are the parts you need:
- '67 to '72 (maybe later) all drum, non-power master cylinder
- 40" (Approx) 3/16 brake line tubing (STEEL ONLY!)
- 1/2" thread, 3/16 tube nut
- 9/16" thread, 3/16 tube nut (on some M/C this is also 1/2" -measure 1st)
- 7/16" thread, 3/16 tube nut
- 3/8" thread 3/16 tube nut (standard size)
- 3/8" thread flare plug
- 3/8" thread flare union