If you are using a vacuum boosted brake system on your g-body, all of the master cylinders will be at an angle because of the angle of the firewall. The plastic reservoir may flat on top, but the cylinder will be pointing up from the firewall at an angle unless your running an aftermarket manual brake adapter that levels out the master cylinder.
Basically the S10 step bore (fast fill) master cylinder housing is the same as the 1982 and up g-body vacuum boosted step bore master cylinder housing. Most of the power boosted versions of the S10 and 1982 and up g-body step bore master cylinders are the same step bores of 24mm (15/16") pressure bore and 36mm (roughly 1.5") volume bore. The manual brake S-10 step bore (fast fill) master cylinder came with a 24mm pressure bore and a 31.6mm volume bore and was also used on some early power boosted g-body cars.
The step bore design (fast fill) is used for LOW drag calipers which require the extra volume of the larger bore to fill the LOW drag calipers. LOW drag calipers retract away from the rotor more that a normal caliper and require this extra volume to push the brake pads back up against the rotor. The step bore design (fast fill) master cylinder is only used on the front brakes (rear port of master cylinder closest to firewall). Front port that goes to the rear brakes will only use the pressure bore size of 24mm. From my experience, any remanufactured caliper is rebuilt to NON low drag (normal) caliper standards and do not require a step bore master cylinder. Any new calipers, that you can find, are also rebuilt to normal, NON low drag standards.
The other difference between the S10 step bore master cylinder and g-body step bore master cylinder is the reservoir. Since the S10 firewall is flat and the g-body firewall is at an angle, there are two different reservoirs to compensate for the different angle and put both lids of the reservoir flat with the ground. The reservoirs are interchangeable between g-body and S10 master cylinders and most likely
b-body step bore master cylinders.
Using a 24mm bore master cylinder with large
b-body calipers and vacuum boosted assist will give you super sensitive brakes with no brake feel and very, very, very easy lockup. You will most likely like need to use a
b-body master cylinder to use with the
b-body brakes to get a good pedal feel and easier modulation for the brakes. If you want to run a step bore (fast fill) the
b-body step bores have a 1-1/8" pressure bore and a 40mm volume bore. Match this master cylinder to the
b-body booster (see conversion to g-body on the web),
b-body front brakes, and you will have more brakes than any tire combination that you can put on your car can handle.
Compared to earlier g-body strait bore (normal) master cylinders (1978-1981), the step bore master cylinders are HUGE in size. All step bore master cylinders came from the factory made in aluminum with a plastic reservoir. ALL new ones are cast iron with plastic reservoirs and as heavy or heavier than the 70s cast iron master cylinders.
Also a booster for an earlier strait bore (normal) master cylinder will not work with a step bore master cylinder as the housing is too large to fit on the earlier booster.
When bleeding step bore master cylinders, I suggest getting a syringe type bleeder to help bleed a step bore master cylinder. The step bore master cylinder has a 100 lbft bypass valve that transitions from the volume bore down to the pressure bore during braking. This bypass valve can hold air and is hard to bleed with the normal bench bleeding procedures. The best way to bench bleed the master cylinder is to use a syringe to push brake fluid back up through the master cylinders outlets up to the reservoir removing the air from the master cylinder's bypass valve. After I use the syringe bleeding process, I then plug the outlets with 1/2-20 inverted flare plugs for the rear port (front brakes) and 9/16-18 inverted flare plugs for the front port (rear brakes) and cycle the master cylinder piston until it is rock hard and does not move more than a 1/16 of an inch. When the piston becomes rock hard and does not move, I hold pressure on the piston of the master cylinder for 45 seconds and see if the pressure holds and the piston does not move. If the piston moves and starts to slowly sink down the bore, you have a bad master cylinder. If find the plug method of bleeding is better than the recirculating hose method because you can test if the master cylinder is good before you bolt it up to the car and bleed the rest of the system.
Hope this helps.
