Brake upgrades Front, Rear, and Master Cylinders for Holden Geminis

Brake upgrades Front, Rear, and Master Cylinders for Holden Gemini’s

One of the more popular small cars amongst the modified car fraternity is the rear wheel drive Gemini. They lend themselves well to big 4 cylinder, turbo, rotary and V6 conversions. However those 250mm solid discs do leave a bit to be desired and most Engineers require a brake upgrade to match the increased power.

There are various brake improvements available, such as Piazza vented rotors, and adaptations to fit early Commodore rotors. Some need thin shrink sleeves under the wheel bearings, and this creates difficulties for the home mechanic as well as the known risk of the bearings not seating squarely or spinning on the spindle. Where ever possible Hoppers Stoppers fit original bearings in the bores of steel conversion hubs, and design the outer diameters to carry the upgrade rotor, making the brake upgrade a bolt on proposition. 

Front Brakes

Hoppers Stoppers upgrades for Gemini’s utilize VT/VZ 296 diameter by 28mm thick vented rotors and twin piston calipers, with original Gemini wheel bearings, seals and grease caps. The holes on the VT caliper cradle bracket need a 0.5mm elongation, and then bolt right on with no brackets required.

The special part is then really the hub, and our ability to source blank rotors to drill them in 4 stud or other special patterns. 5 stud brakes in Commodore pattern use standard VT rotors. On full lock the caliper will just touch the lower control arm, to stop this we suggest you weld a 6mm block against the lower control arm where the steering arm touches, this slightly reduces the turning circle. 15 inch or bigger wheels are needed for this conversion.

One small think to watch is a small increase in wheel offset, as the hub face is now 10mm further out per side. Some find this helps with issues with wheels touching the shock absorbers, but if clearance is tight between tyres and mudguards the change might be of concern. If you haven’t bought new wheels then factor this into your plans. As a rule we find 15 X 6 standard Commodore offset wheels with 195/60-15 tyres clear nicely.

Rear brakes

A lot of Gemini owners do not realize that their standard rear drum brakes are in fact 4 stud versions of the Torana and Commodore rears used at that time.
The shoes, wheel cylinders and all spring and clips are identical, and Holden interchangeability being what it is, early Gemini’s used the same basic parts as Torana’s (incl V8) up to when the Commodores were released and then Gemini and Torana got the Commodore rear drum design, as used up to VL.

The differentials are all in the same family, and use the same wheel bearings, it is therefore easy to have a VL axle shortened and resplined, and thus convert to 5-stud rear.

Better still the Commodore rear disc brake assembly fits the Gemini axle housing ends, the only other difference being the wheel bearings sets are different between drum and disc rears, so you can have a Commodore axle shortened, fit disc brakes bearings, bolt on the full handbrake housing, and slip on Commodore rear discs and calipers. A bit of a fiddle to make the Gemini handbrake cable connect and its all done. 

To do a 4-stud disc rear you can simply change the wheel bearings on the original axle to the disc type and bolt on the rest. This of course needs us to drill from blanks some 4-stud rear discs.

All this assumes you will not have issues with diff center strength, as the Gemini’s use a 23 spline axle. Anything stronger means a full diff upgrade such as a 28 spline Commodore conversion.

Master cylinders

Any big brake kit needs a bigger bore master cylinder; we recommend a 1-inch tandem master be used with VT brakes. How to go about this depends on whether you engine conversion requires relocation of the booster.

Gemini’s came in three basic types; the early TX cars used remote fill reservoirs and a four-bolt mounting between master and booster. If you can keep the booster then we have a 1-inch remote fill master (JB1466 See Picture Below) that is a straight bolt on.

Later TC to TE cars had a horizontal two-bolt arrangement. Again if you can keep the booster we find an XB Falcon master fits, but you need to extend the output pushrod length to correctly preload the master and repipe to 3/8 ball flare fittings, or use 3/8 to 10mm adapters.

The TF and TG series used an angle bolt master cylinder with a plastic reservoir, and to date we have not found a 1-inch replacement for this unit.
You therefore have to fit an early booster to have a firewall fitted unit on this series.

V6 Commodore conversions VN to VP seem to need total removal of the booster, but it seems that VS and later Ecotech engines are narrower and clear. If you have to go to a remote VH40 booster we have a neat 1-inch bore tandem master, which suits this engine type. If going to four wheel discs you will need two remote VH40’s. Don’t forget that you should retain your brake fail warning light and that the original proportioning valve is the wrong pressure for rear discs. A HZ four-wheel disc unit will be right (now obsolete new) or an adjustable unit needed.

Some cars made in the 70’s worked acceptably with no boosters at all, albeit with higher pedal efforts. In fact rare poverty pack Gemini’s were sold without boosters. I remember one car coming in for a service, after a test drive was about to ring the customer to tell her that her booster had failed only to find it never had one! Bigger calipers and rotors of course stop better, so as long as you use soft high friction pads acceptable brakes can still be had with no booster. Not the thing to let your slightly built girlfriend drive.

We hope all the above is a help in getting your Modified Gemini safely on the road.

Peter Koning

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