CELEBRATING 20 YEARS!

Im sure some of you have read this on another forum but the answers have been slow and I need a fix.
I replaced everything in my 81 Malibu. Master, calipers, proportioning valve, rubber lines and wheel cylinders. The wheel cylinders are 1" bore that came with the Torino 11" drum brake in the Moser 12 bolt rear.
I also replaced the booster with a generic dual diaphragm instead of the single that came on the car.
Ive bled the master and the whole system multiple times. The pedal is still soft and goes almost down to the floor. The car is not running. Repeat ! Its not running.

Two weeks ago I finished doing the complete brake system on my 83 Malibu wagon. Again, this car is also not running. The pedal is much harder and doesnt go down as far as the pedal in the coupe. Not even close.

I spoke to someone in tech support at Master Power Brakes. The guy there said that the issue is probably the fact the wheel cylinder bore is 1" and the original rears were only 3/4". He said I would need a bigger bore master cylinder. At least 1 1/8 instead of the original 1".
Guys on the forum said thats incorrect.
Other replies said I might have the wrong pin in the booster. Too long and its not letting the correct amount of fluid into the lines for the rear.

Anyone else have a problem like this ? Im sure someone on here put in a Moser rear with the Torino drum setup in their Monte. Should be the same in all G body cars right ?
Anyone have an answer ?

Are the rear brakes adjusted properly??

Yes they are.

If you're gonna go through all that trouble.. why not run discs?

I didnt have the extra cash to lay out for the disc setup. Two cars are getting expensive to finish. LOL

Did you bench bleed the master cylinder before putting it in the car? Has to be bench bled . Hope that helps.or there is a small leak somewhere not noticed . Good luck

A bigger bore M/C is incorrect, you need to go back to the original size wheel cylinders!

The G's stock bore rear wheel cyl were 3/4". An upgrade to that was the S10 7/8" wheel cyl which the stock G master cyl worked fine with. The stock master on the G and 1990 S10 is a 24mm/36mm bore, The S10 uses the same size master as the G.

One of the masters the 87 Camaro with rear disc uses is a 25.4mm/36mm master. The brake line outputs are on the opposite side of the master. They are inboard instead of outboard like the G master. the F master could swap in but the would require brake line rework.

From looking at rear wheel cyl for the 1970 Torino there was also a 29/32" bore along with the 31/32" bore which appeared to be the common one.
Maybe downsizing the wheel cyl could be done to better match the G master to the Torino drum brakes.
Bob

Going to a larger wheel cylinder should not be an issue as the volume of brake fluid needed to fill them is not much.

A 3/4" bore wheel cylinder has a bore area of 0.44 square inches
A 7/8" bore wheel cylinder has a bore area of 0.60 square inches
A 1.0" bore wheel cylinder has a bore area of 0.79 square inches

Though the 1.0" bore has 78% increase in bore area, the amount a fluid to fill either is still relatively small.

Compare a wheel cylinder with a typical rear caliper size of 48 mm (1.9") bore. A 48 mm bore has a bore area of 2.8 square inches.

That is a 257% increase over a 1.0" bore wheel cylinder. This is a dramatic change in brake fluid volume needed.

Usually GM stepped up the bore size when going to rear disc brakes. Usually a drum brake rear got a 24 mm bore master cylinder a a rear disc when to a slightly larger 1.0" bore master cylinder.

Most all GM vehicles from 1982 to about 1997 used a step bore (dual bore) master cylinders. The step bore was used for FRONT LOW drag calipers from 1982 to 1997. Rear calipers or wheel cylinders where not designed as LOW drag.

LOW drag calipers are designed to pull the caliper piston into the bore slightly more that a normal, NON low drag caliper. This effectively pulls the brake pad away from the rotor reducing drag.

Because the piston is pulled into the bore slightly more, it takes more brake fluid volume to get push the piston out more and get the brake pad against the rotor. This is why a step bore master cylinder is needed.

The step bore master cylinder has two bore sizes. A larger volume bore and a smaller pressure bore.

The larger volume bore is used to fill up the front low drag calipers to get the piston push out and the brake pad back against the rotor.

When the pads contact the rotor, pressure builds in the system.

When the brake line fluid pressure builds to about 100 lbft,, a built in pressure valve in the step bore master cylinder transitions from the larger bore to the smaller pressure bore to build the pressure needed generate the clamping forces for the front calipers to stop the car.

The rear port (closest to the firewall) of the master cylinder goes to the front brakes is the only port used for the step bore function of a step bore master cylinder.

The front port (closest to the front of the car) of the master cylinder goes to the rear brakes and does not use the step bore feature. It will always use the smaller bore of the step bore master cylinder.