I tried to upload the full article I wrote on this but It's a bit much. I'll post it in pieces instead.
I’ve been doing some brake upgrades while I did some motor upgrades. It passes the time while I wait for parts. A little while back I happened to have the luck of getting my hands on a complete upgrade set for the rear brakes. This included all hardware and fasteners to install a set of Isuzu rear calipers and rotors to my 72 GT. It did mean replacing the bearings, keepers and a reshim of the new axle bearings. The axle bearings weren’t needed but seemed like a good idea while I had it apart. This included mounting plates that were installed instead of the bearing retainers. These were from an early model Turbo Isuzu and will fit inside of 13 inch rims. Once installed I had very weak brakes and after doing a bit of math on cylinder sizing I found out why. The original rear cylinders are about a third smaller in volume than most calipers even the floating single sided ones from the Isuzu. I was faced with going back to the drums, restrict the fronts or come up with a new idea. I chose the new idea approach and used a 82 master cylinder from a BMW 528 I. It was made by ATE and looked to fit and used all the same sizing as an Opel for lines and banjo bolts. For that year BMW there are a couple used and I chose one that powered 4 wheel discs already and had a single front and rear outlet. I didn’t try the booster at the same time but it looks to be a good donor as well. It has front and back bores of 24 VS. the 17 or so from a stock GT master cylinder. I did have to split the fronts using a rear axle tee fitting. I was right and it was the same length as an Opel MC but had the outlet ports on the wrong side. I took it apart to see how it operated and check it for usability as it was a used part. It has much larger cylinders and the bias is set by 2 springs that can be changed to get the bias right if needed. I had some work to install it as the pictures show but it does fit. I had to cut some of the drivers side light bucket opening and use a line for the fronts turned as quick as possible to clear everything. I did use a banjo bolt on the back but to get it to turn and tighten I had to round off the corners of the square attaching bolt. An Opel reservoir doesn’t fit this master cylinder and a BMW one won’t clear the front sheet metal. I cut the BMW one at the full line for the forward half and reattached a new top for it made out of plastic, using Seal All. It did just that and has held up to the brake fluid as well. The function is exactly as planned. I’ve tested this at varying speeds up to 80 MPH and it is extremely firm and the nose dive or front lock up is no longer there. I do believe that this will allow for bigger front brakes to be used with no loss of pedal. I’ve also recently found out that BMW 320I front calipers will fit the GT if swapped side to side and quite possibly the stock non vented calipers may work as well. These would be from a 77 or so model I’ve been told. I’ve located a full used set locally that I’ll pick up as soon as the put the car out for parts. If the BMW front calipers and rotors do fit and only need pads and some light work my total cost for the complete upgrade will be around 275.00. I’ve been extremely lucky with parts on this and I know I could never hope to repeat it at that cost. This wasn’t the easiest upgrade but maybe the most important. If it won’t stop then what fun is going fast?
Parts list
Isuzu rear calipers with rubber lines
Emergency brake cable with hardware
Custom mounting brackets for the calipers
New Isuzu rotors
New brake pads
Axle bearings
Axle bearing retainers
Used Master Cylinder with reservoir
Rear axle Tee and short steel line from a stock caliper
Used front calipers and rotors
I’ve been doing some brake upgrades while I did some motor upgrades. It passes the time while I wait for parts. A little while back I happened to have the luck of getting my hands on a complete upgrade set for the rear brakes. This included all hardware and fasteners to install a set of Isuzu rear calipers and rotors to my 72 GT. It did mean replacing the bearings, keepers and a reshim of the new axle bearings. The axle bearings weren’t needed but seemed like a good idea while I had it apart. This included mounting plates that were installed instead of the bearing retainers. These were from an early model Turbo Isuzu and will fit inside of 13 inch rims. Once installed I had very weak brakes and after doing a bit of math on cylinder sizing I found out why. The original rear cylinders are about a third smaller in volume than most calipers even the floating single sided ones from the Isuzu. I was faced with going back to the drums, restrict the fronts or come up with a new idea. I chose the new idea approach and used a 82 master cylinder from a BMW 528 I. It was made by ATE and looked to fit and used all the same sizing as an Opel for lines and banjo bolts. For that year BMW there are a couple used and I chose one that powered 4 wheel discs already and had a single front and rear outlet. I didn’t try the booster at the same time but it looks to be a good donor as well. It has front and back bores of 24 VS. the 17 or so from a stock GT master cylinder. I did have to split the fronts using a rear axle tee fitting. I was right and it was the same length as an Opel MC but had the outlet ports on the wrong side. I took it apart to see how it operated and check it for usability as it was a used part. It has much larger cylinders and the bias is set by 2 springs that can be changed to get the bias right if needed. I had some work to install it as the pictures show but it does fit. I had to cut some of the drivers side light bucket opening and use a line for the fronts turned as quick as possible to clear everything. I did use a banjo bolt on the back but to get it to turn and tighten I had to round off the corners of the square attaching bolt. An Opel reservoir doesn’t fit this master cylinder and a BMW one won’t clear the front sheet metal. I cut the BMW one at the full line for the forward half and reattached a new top for it made out of plastic, using Seal All. It did just that and has held up to the brake fluid as well. The function is exactly as planned. I’ve tested this at varying speeds up to 80 MPH and it is extremely firm and the nose dive or front lock up is no longer there. I do believe that this will allow for bigger front brakes to be used with no loss of pedal. I’ve also recently found out that BMW 320I front calipers will fit the GT if swapped side to side and quite possibly the stock non vented calipers may work as well. These would be from a 77 or so model I’ve been told. I’ve located a full used set locally that I’ll pick up as soon as the put the car out for parts. If the BMW front calipers and rotors do fit and only need pads and some light work my total cost for the complete upgrade will be around 275.00. I’ve been extremely lucky with parts on this and I know I could never hope to repeat it at that cost. This wasn’t the easiest upgrade but maybe the most important. If it won’t stop then what fun is going fast?
Parts list
Isuzu rear calipers with rubber lines
Emergency brake cable with hardware
Custom mounting brackets for the calipers
New Isuzu rotors
New brake pads
Axle bearings
Axle bearing retainers
Used Master Cylinder with reservoir
Rear axle Tee and short steel line from a stock caliper
Used front calipers and rotors
