A while back I ditched the leaking vacuum booster and slapped on a Mopar manual MC, using a custom pushrod moved up to the upper hole in the pedal. The brakes 'work' - all 4 wheels brake like they should, stops straight, balance seems OK, but the pedal effort is INSANE. IIRC, the Mopar MC is 15/16" bore, which again IIRC is the same as the factory S-10 manual MC (could be wrong though, been a long time since I checked). Long time ago I had an S-10 with manual brakes from the factory, and the pedal feel wasn't anything like this. You'd barely even know it wasn't a power setup. Do I need to hunt down a 7/8" MC, is that enough of a difference to make a dramatic change in the pedal effort? Anybody got any suggestions on where to start?

 

I have been working on my own manual brake setup for my 1978 Malibu. I am planning to run the stock 7/8' bore master cylinder made for a manual brake equipped g-body. It is cast iron, but it is made for a g-body. Up in size from this, is the stock vacuum assisted master cylinder from g-body and they are 15/16" bore. The earlier ones were cast iron and the later ones where aluminum. There is also a 7/8' bore master cylinder that comes out of a 1993 Dodge Spirit. It looks to have the correct angle for mounting it on the firewall. There are two issues that I know about. First, the distance between the bolt holes on the dodge master cylinder are a little narrower than a gm master cylinder. Second, the outlets for the brake lines are a different size and adapters will have to be used (I believe this is correct for all dodge master cylinders). I think there is a disc/drum (10 lb residual valve installed) and a disc/disc (no residual valve installed) version

There is some good information on the pro-touring.com website. These guys are converting to manual brakes for racing to get a better feel of the road while braking. They seem to have a better idea of what is involved in converting to a manual braking system.

Dave

 

Thanks! I found the G-body manual MC a while back but wasn't sure on the bore size, I think I'll give that one a try before I resort to switching back to vacuum.

http://shop.advanceautoparts.com/we...Brake-Master-Cylinder-Cardone_5351669-P_230_R

 

http://www.mpbrakes.com/technical-support/troubleshooting.cfm

If you have drum rear brakes, make sure you have an inline 10 lb residual valve for the rear brake lines.

What did the mopar master cylinder come out of? Most of the popular ones have a bore size in excess of 1". This would make a hard pedal.
What is your brake set up?

Dave

 

Stock brake system except for the manual MC, everything worked fine before the booster sprung a vacuum leak. It's the smaller of the two 'universal' ones that used to be common from Mopar performance, so it should be the 15/16". I don't see them listed anywhere for sale now, and I had it for years before I finally used it and the box is long gone so I don't have a part number. Like I said, brake system behaves as it should except for the pedal effort - bleeds out fine, stops straight, etc., I'm just afraid I'm gonna pull the steering wheel off the shaft trying to get the thing to stop!

 

I haven't measured it, but I'm using the upper hole (power setup used the lower hole). If the ratio is wrong it will just have to work like that or go back to vacuum, changing it would mean building a new pedal & bracket from scratch and I'll just buy another 8" dual booster before I do that.

I think the deal is that I replaced a 15/16" power MC with a 15/16" manual MC, and while moving the pushrod to the upper hole helps some it's just not enough.

 

For a stock g-body, the pedal ratio for a manual brake car was a 6:1 ratio (upper hole), I think you are good on that. What does matter is that you have a 10 lb residual valve for your rear drums. What the residual valve does it keep some pressure on the rear drums so it doesn't fully retract after you depress the brakes.

I haven't tried these master cylinders yet, but they are dodge style master cylinders with different bore sizes. Dorman is the one that had the bore size listed and that is why I included them. Cross reference those part numbers to any other brand. From the pictures, they look like they have an angle to them to match a g-body firewall. There are adapters to mount the mopar brake line size in the master cylinder to the brake lines of the g-boby.

Mfr PN Bore Measure Inches (decimal) Brakes Notes
Dorman M39451 21 mm 0.827 Disc/Drum 1993 Dodge Shadow w/Power Rear Drum Brakes;
Dorman M39736 7/8 in 0.875 Disc/Drum 1993 Dodge Spirt ES V6 w/Power Rear Drum Brakes;
Dorman M39476 24 mm 0.945 Disc/Disc 1993 Dodge Spirt ES V6 w/Power Rear Disc Brakes;

 

I converted my Malibu when I changed to turbo. I have used stock power MCs and stock manual MCs and I can't tell a nickels worth of difference between them. I finally just went and robbed a booster off of a friends regal and called it good.

 

I used a fresh GM iron "don't know what it came on" mast cyl on my '71 C-10 when I went to manual. It was 7/8" bore. Stock truck dist block, stock '71 disc front, drum rear.

Hard pedal, VERY high effort, could not dream of locking the wheels up. Swapped on an iron '73 nova mast cyl disc, manual front disc, rear drum brakes , 15/16" bore, all is well.


btw, I looked at all of those mopar options. Note: some have really goofy threads for the lines. Iirc, Autozone's site lists threads on them.


The nova mast cyl fit, as far as mounting and lines, but i had to modify rod under dash.

Dave

 

After some research, I think the biggest problem when running manual brakes on a g-body is the front calipers. There were slight variations on g-body calipers depending on the years. All calipers from 1978 to 1987/88 look the same, but from about 1981 on, most calipers were quick take up, which means when the brake pedal was released, the pads backed away from the rotors for less friction and rolling resistance. GM did this to try to increase gas mileage. To compensate for the quick take up calipers, GM designed a step bore master cylinder. A step bore master cylinder had a larger bore and a smaller bore within the same master cylinder. The larger bore would increase the volume of brake fluid to the front quick take up calipers, and the smaller bore would be used to apply the pressure to the front quick take up calipers once the larger bore got the pads to the rotors surface.

All front brake calipers look the same on all g-body vehicles. They can also be interchanged. S10 trucks and S10 blazers also used the same calipers until they quit production around 2002. Camaro and Firebird used these calipers from 1982 to 1992 on the third generation F-bodies. This means the same caliper was used for over 20 years and most of these calipers where quick take up, which means the odds of any rebuild caliper would be quick take up also. What do you have on the front of your car is more than likely quick take up calipers which means you would need a step bore master cylinder if using stock front brakes.

For manual brakes on a g-body using stock front and rear brake setups I would mimic the S10 set up. The only change I would do is oversized front calipers and good aftermarket brake pads. I would run a new S10 manual brake master cylinder (24mm/31.6mm step bore for the S10 v 24mm/36mm for power brake g-body from 1981 and newer v 15/16' for g-body power brakes from 1978 to 1980 v 7/8' g-body manual brakes from 1978 to 1980) with the g-body reservoir swapped over to keep the angle correct, oversized aftermarket front calipers (2.75' bore for the aftermarket caliper versus 2.5' bore for the G-body), S10 manual brakes rear wheel cylinders (7/8' bore for the S10 versus 3/4' bore for the G-body), and good front performance brake pads (EBC, Hawk) that work great when cold.

The step bore on the S10 manual master cylinder will increase the volume of brake fluid to the larger front calipers and rear wheel cylinders with the primary 31.6mm bore and increase the pressure to the brake pads and shoes with the 24mm secondary bore. It will give you good pedal feel and increase braking ability.

 

I am trying to relocate my master cylinder to the inside of my 1968 chevelle and have a wilwood pedal assembly and master cylinder and i am have a hard time trying to figure out how to mount it. I have to move it because of the turbo exhaust on the drivers side will be against the master cylinder if i dont move it. can anyone help me with this problem?