kjbuster1

All - working on my ‘56 with a Jim Meyers front suspension and 4 wheel disc brakes. Front discs are 1970 Chevelle single piston and rears are single piston Cadillac with emergency brake lever. Due to engine compartment limitations it is a manual brake system and for simplicity reasons would prefer to keep it that way. The master cylinder we are using is a 1971 Corvette disc/disc manual from Advance Auto with a 1 inch bore. Car isn’t stopping as well as expected and we pressure bled the system and are confident there is no air in the system. We bought a brake pressure gauge and front brakes are showing approximately 800 pounds of pressure rather than the 1,200 we were expecting. Any suggestions regarding what is causing the issue and how to resolve?

we are leaning towards using Disc/Disc Manual master cylinder with a 1 1/8 bore, but not sure if this will help. Thanks for any advice. Eric

adamsocb

Pedal ratio is also a factor. Are you using an original pedal with original pivot and MC attachment points?

Also, you are contemplating going the wrong direction. To increase the line pressure you need to reduce, not increase the MC bore. To go from 800 PSI to 1200 PSI, you will need a 3/4”-13/16” bore depending on the pedal ratio and leg force you are applying. This will also increase the pedal travel.

The basic formula for brake line pressure is:
Leg force in lbs. times the pedal ratio (PR), divided by the area (A) of the MC bore in sq. Inches. Remember the area in sq.in. (A) is half the MC bore diameter squared times pi. (Pi times the radius squared)

PSI=(lbs. x PR)/A

There are many pedal ratio and pressure calculators on-line. Just search for “brake pressure calculator”

Charles

kjbuster1

Thanks for the detailed response. The master cylinder was in the car when I bought it, but pretty certain it is original pedal set up. Regarding MC attaching point there is an extension on it to move it away from the firewall I believe to gain clearance for brake lines. See attached picture.

adamsocb

The stock 56 Corvette brake pedal ratio (PR) is 5.65:1. The extension adapter doesn’t change the PR.
I did some checking and don’t see a tandem MC smaller that 7/8” available.
7/8” and 15/16” bore MCs are generally for manual brakes. 1” and up are for power boosted.

I don’t know if the OE type tandem MCs are available in 7/8” or 15/16”. You may have to go with Wilwood or Baer.

Another variable is the pads. Good performance pads will provide much better braking with less pressure than OEM or inexpensive auto parts store pads.

The last resort in an electric remote power system. I understand these work great, but they are pricey at ~$1500 for the electric booster and special MC.

I will have Wilwood 6-piston fronts and 4-piston rears on my 1956 Art Morrison chassis. I haven’t finalized 7/8” or 15/16” MC yet. If I were the only driver I would probably do 15/16” to reduce the pedal travel, but since my wife will be driving it, I should do 7/8” to reduce the effort. If manual doesn’t work out I will do the electric boost.

woodrow2002

Color me a little surprised, but looking through the forum, several other users have reported installing a similar system using stock pedal & master cylinder (as I understand, all stock C1 single master cylinders are 1" bore) and having no issues. As I recall, for the Caddy rear brakes, you have to actuate the parking brake a few times to adjust the rear brakes (there is no auto adjusting on these rear brakes), failure to do that and you won't have effective braking for the rear (which might be your problem). You may have to check that your parking brake lever is capable of fully actuating the parking brakes themselves (sometimes there is an issue as there may not be enough pull on the cable as the design is intended for drum rears). As a side note, it seems you have plenty of space in the engine bay, several major corvette suppliers do have a dual master cylinder with power booster that can replace what you have.

dplotkin

You will never have brakes with 800 PSI. Adding power assist will just make it easier to not stop.

I agree that rear brakes have an effect on pedal height. You need pressure and volume. As mentioned above, its all in the math. Don't fire a parts cannon at this, sit down and figure out how much fluid, and at what pressure you need to push through all the bores on those 4 calipers. Disk brakes need LOTS more fluid to move the pistons than drum wheel cylinders need to move shoes.

I have been through hell myself making front GM disks work on an early 60's Mopar retaining its original giant rear drum wheel cylinders on tapered axles that don't cooperate with mods.

Its all in the math.

Dan

adamsocb

I missed the parking brake lever on the rear calipers.
I agree with Woodrow, the parking brake mechanism screws the piston in to adjust for pad wear. If you haven’t checked the parking brake, make sure it will lock the rear wheels from the hand brake lever.

solidaxel

I'm working on making a hydro boost work
that requires power steering pump and it requires lot of real estate
on the drivers side

adamsocb

This is my alternate plan if the manual setup requires too much pedal force to drive comfortably.
Electro-hydraulic Power Brake Booster
As I said before, it is pricey at ~$1,600, but the MC will fit for sure and the electric booster unit can be located anywhere. The description says it will generate over 1600 PSI line pressure. I talked to a guy at a car show who had one and he said it worked great.

vjjack04

AdamsOCB,
I and several others with C2 "restomods" have used this the electric booster. It works great. I have also started seeing them in other cars at car events. However, there are also several folks I talked to that use a complete manual Willwood system with their master and 6 piston front and 4 piston rear brakes and they work very well.

adamsocb

Thanks,
I agree on the manual. That’s why I will try it first. Both Wilwood and AME technical advisors have told me manual should work fine. The electric boost is my backup plan if my wife complains. 😇

vipervetteguy

I’ve used this system in several cars now. It’s in m 66 resto that has C7 Z51 brakes. It works great and the MC is a work of art!

Sunstroked

Heres what I have found. I have a 65 Mustang GT that I built for my wife. It came with stock disc front, drum rear. And a single reservoir master. I wont ever run that on a road driven car. My 72 Corvette restomod uses a Wilwood dual master, a hydroboost and wilwood brakes calipers. Well on a trip to a car show, I had a front brake line failure and thankfully the rear brakes got me stopped. (my fault, I used too long of Wilwood flex brake lines and they rubbed on the tire, eventually right through). Harrowing? Oh yeah. So I put a 1967 Mustang dual master cylinder on the 65 GT. Not being power the 7/8" mc has enough brake pressure to stop the car. But it really requires more pedal force to stop and hold. Really noticable at traffic lights. You got to keep pressue on the pedal. Food for thought. Im going to switch to a vacuum power booster for the 65. All about drivability for my wife and I.

dmtparker

As an alternative, you might look for the booster from a Jaguar XJS. My '91 has a booster that looks VERY similar and it works great.

solidaxel

I have not looked into what all of the E V are using
the master cylinder would be a concern
just thinking!

vjjack04

Adamsocb,
If you think it will be someting as a final solution, i recommend you go ahead with the electric system. Stops as good as my wifes 2020 BMW X5 and if the electric boost fails it goes to manual brakes which would stop just fine....you never have "no brakes".....unless of course some catastropy happens....as with any system.

kjbuster1

Late last week we switched to a 1970 Corvette master cylinder for 4 wheel disk brakes with a 1 1/8 bore. Significant improvement as rear brakes now lock up under hard brake pressure, but still not performing as well as expected. Ordered some new yellow EBC brake pads for front brakes. Will install on Tuesday and provide additional update.