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I recently was presented with the challenge of adding a disc-brake rear axle to friend's Ascona. The straighforward installation of the axle is simple, the problem that ensues when using rear disc brakes on a car not designed for it is the front-to-rear brake bias. Most master cylinders for front disc/rear drum applications are biased at approximately 65% front and 35% rear, because of the volume of fluid required to activate the large pistons in the front calipers compared to the smaller pistons in the rear cylinders.

Adding rear disc brakes requires a larger volume of brake fluid to be sent to the new, larger caliper pistons. Not doing so will make the pedal 'lower' than usual, as the normal fluid volume that is sent to the rear cylinders only partially applies the clamping force to the new rear caliper pistons. Numerous pumps are necessary to get a firm brake pedal. Adding a brake proportioning valve is not the answer....they only reduce the amount of brake fluid, not increase it. So the answer is to install a master cylinder with a greater rear bias. Not a problem if your car was available with an optional rear disc setup, just order it from the dealer, right? Yea, sure, we're talking Opels here folks!

For the Opel Manta, I haven't found anything that is a direct bolt-up for rear disc brakes. And to be honest, the brake booster in my friend's car was starting to stick anyway. So instead of replacing the master cylinder (usually about $150-$175 for a stock replacement anyway), and rebuilding the booster
($100 to $150), I said 'screw it' and built a custom adjustable brake pedal assembly for his car, eliminating the brake booster altogether. The trick was to make a custom pedal asembly but retain the stock clutch pedal arm....but there's not a whole lot of room under there to do this. So I just modified a stock Opel Manta/Ascona pedal cluster, retaining the stock clutch arm location and adding the adjustable balance bar pedal assembly and twin racing master cylinders. Total cost was just over $200 for the twin master cylinders (7/8" front and 3/4" rear), and the pedal assembly from Wilwood. I went with the 12" pedal length to increase the leverage ratio so the pedal effort wouldn't be too difficult.

Bob


For comparison, here's the stock pedal assembly:
 

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Discussion Starter · #2 ·
And here is the modified pedal assembly. Yes, I lightened the clutch arm a little bit, and overall saved about 25 lbs compared to the stock pedal assembly, cast iron master cylinder, and brake booster.
 

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Here is the other side of the pedal assembly, showing the twin aluminum master cylinders. Plenty of fluid capacity, and easily rebuilt if needed. Also, NO RUST!!
 

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While we're talking about GT pedal clusters...are they removable? There are four bolts around the brake rod bellows but even once you remove these it doesn't seem to budge. Are there bolts from the other side that I didn't see or is this tack welded in place? My back REALLY doesn't like it when I'm looking around under the dash so I didn't spend too much time looking.

-Travis
 

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The GT doesn't have a pedal cluster like that, it's basically just a pivot point welded to the firewall and the pedals run off of that.

Great job on it Bob, I was looking at doing something like that, but my Dad kept telling me that I had to have a brake booster or else it would be way too stiff for braking. I'd like to know how much stiffer it is, this is a much better way to go I think.
 

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Discussion Starter · #7 ·
Pedal effort will be affected by the pedal length (leverage) and by the master cylinder bore diameter. Going with a much larger bore will increase the pedal effort, while simultaneously reducing the distance the pedal has to travel. A smaller bore master cylinder will require less effort, but will require greater pedal travel. Going to extremes, if the bore was too small, it would bottom the piston in the master cylinder before the pistons at the calipers were fully extended (insufficient fluid volume).

I've used manual brakes before in Opels, the effort is higher, but also easier to modulate, especially in threshhold braking. Brake boosters dampen the pedal movement, and are slower to release when you pump the pedal, creating slower caliper retraction. So if you lock a tire, it takes longer to unlock it with power brakes. And if you run a radical cam in your Opel, you already pretty much have no power brakes anyway from the lack of engine vacuum!!

Bob
 

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do you think a m/c for a chevy s10 blazer by itself would be too much as far as peadl feel on an opel
i plan on using disc al the way around with fiero setup and 3rd gen camaro disc rear
 

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RallyBob said:
And here is the modified pedal assembly. Yes, I lightened the clutch arm a little bit, and overall saved about 25 lbs compared to the stock pedal assembly, cast iron master cylinder, and brake booster.
Any plans on how to do this or can you make me a setup like this.. ;)
 

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Discussion Starter · #11 ·
GoinManta said:
Any plans on how to do this or can you make me a setup like this.. ;)

I have no plans for it, I built it as a one-off. And you know my stand on anything brake or suspension related...
 

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I recently was presented with the challenge of adding a disc-brake rear axle to friend's Ascona. The straighforward installation of the axle is simple, the problem that ensues when using rear disc brakes on a car not designed for it is the front-to-rear brake bias. Most master cylinders for front disc/rear drum applications are biased at approximately 65% front and 35% rear, because of the volume of fluid required to activate the large pistons in the front calipers compared to the smaller pistons in the rear cylinders.

Adding rear disc brakes requires a larger volume of brake fluid to be sent to the new, larger caliper pistons. Not doing so will make the pedal 'lower' than usual, as the normal fluid volume that is sent to the rear cylinders only partially applies the clamping force to the new rear caliper pistons. Numerous pumps are necessary to get a firm brake pedal. Adding a brake proportioning valve is not the answer....they only reduce the amount of brake fluid, not increase it. So the answer is to install a master cylinder with a greater rear bias. Not a problem if your car was available with an optional rear disc setup, just order it from the dealer, right? Yea, sure, we're talking Opels here folks!
:

Don't Opels already have a proportioning valve in the brake line before the rear brakes? I can't remember, it's been so long since I've had a stock Opel. Most cars with rear drum brakes do. The answer is if you're fitting rear disc brakes you should be able to remove the stock proportioning valve and fit an adjustable one and still run the stock master cylinder. My Ascona B rally car has a disc brake rear with nice AP calipers. Since the brake lines were all plumbed inside the car there was no proportioning valve when I got it. It used the stock master cylinder. It had too much rear brake bias when I got it, very heavy. You want more rear bias on a rally car than normal but it was too much for my liking so I installed an adjustable proportioning valve to dial some of it out. I even tried a GT master cylinder and it was just the same so it's not like it had some funny master cylinder in it. Anyway, now I'm upgrading the front brakes so I'm doing a dual master cylinder/ adjustable bias bar like Bob did. I was never happy with the brake feel before.
But my point is you shouldn't have to go to all that trouble and expense to convert to rear disc brakes if you don't want to.
 
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