FE-FC Holden Discussion Forum

This is really for Ken but some one else might be able to help A while ago if I am correct  you said that Hopper Stoppers use the Falcon AU and BA calipers in preference to the dual piston Commodore as the Commodore  calipers needed a Commodore master cylinder to operate correctly and when used with out this master cylinder the brake pedal gave a spongy feel am I on the right track ???? what size piston is used in these Commodore master cylinders ???? Thanks .....FC427.......

Your on the right track but the full answer takes a little explaining so here goes.
The Commodore master cylinder has a "step bore" - What this means is that the piston has 2 sizes, a larger diameter which I am not sure of, but I think is about 30 mm diameter, and a primary diameter of 25.4 mm or 1 inch. This means that when you put your foot on the pedal, the slack in the system is taken up quickly by the larger piston diameter and once the load is achieved the 1 inch bore takes over and this is the active size.
The other issue with Commodore calipers is that they have a seal which acts to retract the piston into the caliper and away from the rotor. GMH did this to try to stop rotor warping which plagued the VT onwards Commodores. So the calipers produce a backed off piston and the master cylinder counteracts with its step bore. If you use a standard 1 inch bore with the Commodore calipers you get a spongy feel until the slack is taken up in the caliper.
The last issue is more of a practical one in that the Commodore caliper mounting bolts are almost the same distance apart as the holes in the HK-HZ stub axle - this means that an adapter plate is difficult because the holes overlap. The AU and BA Falcon calipers on the other hand have a very wide mounting hole position which allows easier adapting plate design. The AU/BA caliper also doesn't have the retractive piston design so a 1 inch master cylinder gives a nice firm pedal feel.
The AU and BA Falcon master cylinder is 1 inch bore and the piston diameters in the Commodore and the Falcon are the same as well, so everything is matching.

So the upshot is that the VB to VS Commodore upgrades which Hoppers Stoppers offers which use VT Commodore calipers have a spongy feel whereas the 330 mm brake upgrades for the same Commodores have a firmer feel because they use AU calipers.
Don't get me wrong in that the brakes are unsafe in any way - once you get used to the feel, the brakes themselves work far better than the original brakes - but obviously if a VT booster is fitted then you will get the same brake feel as a VT

Hope this makes sense
Ken

Thanks Ken All makes sense. Would 2lb or 4lb  residual valve help this spongy feel ??? It is for my 27 T roadster and I have made a pedal assembly to fit under the dash with the master cylinder from a Mitsubishi van which is only 15/16 bore and only the front circut is boosted with a remote VH44B  booster and I use Vt calipers front and drum brakes rear will this make my pedal even sponger than with 1'' master cylinder and produce to much travel in the pedal ????...FC427....

The residual valve is not a great idea with discs, the pads should retract off the rotors in a disc brake system and the residual valve will stop this happening. Residual valves should only be used with drum brakes.
What is your pedal ratio? I assume it is the stock Mitsi under dash unit you have put in, My feeling is that the brakes will feel spongy and another factor is the VH44 is not designed for front discs - it is really a drum brake booster.

Ken

Ken the pedal ratio is 7.5 to 1 the pedal assembly is one I have fabricated using part BMW . I thought residual valves for drum brakes were around 6lb and hoped that 2lb might have compensated for the two stage piston on the Commodore thats why I asked  ??? Thanks....FC427......

Ken I looked up www.hydrabooster.com they are the people that I bought my booster from and on their web they state that the VH44b has more power than a VH44  and it is suitable for disc brakes is this wrong ??? also I am only boosting the front circut . Will go to the brake shop and see if I can get a larger dia master cylinder with the same mounting flange for it . Thanks ...FC427....

I'm not sure about VH44B - they are probably correct since they sells the boosters. We have always used VH40's for disc brakes at Hoppers Stoppers. If you had a shorter pedal ratio of say 6 to 1, then you would get more distance from the pushrod for the same amount of pedal travel. The brake effort will be higher, but the booster will help this.

I don't think the residual valves will work but am keen to find out your results if you try them.

Ken

Ken I have done a little research and have come up with * The only time that a 2 lb residual valve on a disc brake system is needed is when the master cylinder is mounted lower than the calipers, this will prevent the fluid from siphoning back to the master cylinder . Quoted from Boyd Coddington's Drivelines & Chassis. I have reduced the pedal ratio to just under 6 to 1 and my mate at Better Brakes is Chasing a Master cylinder with a bigger bore and the same mounting flange but still with a remote reservoir......FC427.......