NEW MASTER CYLINDER OPTION

TURNA

Moderator
Jul 6, 2003
6,031
0
36
Socialist NY
This master bolts right in no mods and actually works

AC DELCO Master Cylinder Part # 89056529 or 18M974 (same master)




 

malibudave1978

Amateur Racer
Jun 8, 2004
226
0
0
Houston, TX
Turna, Thanks for the heads up on this master.

These later model S-10 master cylinder are great, but there are some things to keep in mind.

The outlets on this s-10 master cylinder are both 1/2-20 inverted flare. (Correct me if I am wrong).
The rear outlet of most g-bodies for the front brakes is 1/2-20, which will bolt up to this new S-10 master cylinder.
The front outlet of most g-bodies for the rear brakes is 9/16-18, which will require a new brake line with the correct fittings or an adapter fitting.

The adapter fitting to mate the stock brake line to this S-10 master cylinder should be either Edelmann PN 258306 or 258307 (I can never tell from the description which is which). This will be the easiest option.

The other thing to keep in mind is that from 1982-1988, the front g-body calipers came from the factory with LOW drag calipers. LOW drag calipers require a step bore master cylinder because the front caliper pistons retract away from the rotor more than a normal caliper. A step bore master cylinder uses a 1.25" volume bore to move more fluid and push the caliper piston back up against the rotor. Once the pads are back up against the rotor and the pressure in the system builds to 100 lbft, then the master cylinder transitions to a smaller 24mm bore for more pressure and substantially greater clamping force.

This S10 master cylinder has a 1.0" strait bore master cylinder. If you still have your stock, original, from the factory calipers, you will need to replace them or you will have a pedal with a longer pedal travel. You will have a longer pedal travel because the 1.0" strait bore master cylinder will have to move more fluid volume to move the LOW drag caliper piston back up against the rotor before it will start to build pressure and caliper clamping force. Any stock replacement, new or remanufactured, caliper should be a NON low drag, normal calipers. All caliper part numbers are the same for 1978-1988 g-bodies. 1978-1981 calipers, from the factory, where NON low drag, normal calipers and they used a strait 24mm bore master cylinder. 1982-1988 g-bodies calipers, from the factory, were LOW drag and used a step bore master cylinder. A strait bore master cylinder cannot be (or should not be) used with LOW drag calipers, but a step bore can be used with NON low drag, normal calipers and LOW drag calipers.

Just keep these things in mind if using this as a replacement master cylinder which is a great alternative.
 

TURNA

Moderator
Jul 6, 2003
6,031
0
36
Socialist NY
There are 2 versions of this master with 2 different part numbers

1 the above part number is a direct bolt in.

2 the one you mentioned is a different part number with the incorrect threads
 

malibudave1978

Amateur Racer
Jun 8, 2004
226
0
0
Houston, TX
TURNA said:
There are 2 versions of this master with 2 different part numbers

1 the above part number is a direct bolt in.

2 the one you mentioned is a different part number with the incorrect threads

Thanks Turna. I learned something new.

I will keep this part number in mind. It can also be used for manual brakes IF the braking system needs it. 99.9% of manual brakes DO NOT need to go above 24mm or 15/16" bore master cylinders. But sometimes a customer will run over sized 2.75" calipers on the front and a stock, metric 2.5" calipers on the rear. This set up needs A LOT of volume to work correctly.

This master cylinder will work great in a power system with NON low drag, normal calipers or the larger, bolt in, big bore calipers.
 

sunnybob

New Member
Jan 17, 2019
2
0
0
Hi, could you confirm ACDELCO 18M974 will fit 1978 Malibu Power booster and brake line with no mods, please? Thank you so much.
Ciao
Roberto
 
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