AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
- Jan 19, 2014
- Messages
- 24,940
- Reaction score
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Ok; if the front brakes are being operated out of the front reservoir, then almost everything I have said may be wrong.
Here's the deal;
Inside your M/C are two pistons linked but operating independently.If you were to disconnect the line to the rear brakes, and then stomp the brakes, this is what goes on; The footpedal is connected to the pushrod, which is connected to the rearmost piston. the end. Pedal goes down, piston moves forward. On the front of that rearmost piston is a Mini pushrod. When the rear piston moves forward the mini pushrod eventually reaches the front piston. If you have enough pedal travel left, the front piston will push fluid out to the front calipers. if you still have pedal travel left the brakes will apply and the pressure will build, stopping the car.
So the two most important things are sufficient pedal travel, and the front piston returning to its rear-most parking spot. Since SSBC in there infinite wisdom swapped the working chambers, rear to front, there is now no mechanical way of pulling the front-brake piston to it's rearmost parking spot.So how are they returning the piston? ima gonna assume there's a spring in there, cuz seal retraction is not going to do it.Moot point cuz we already know it's returning by the fact that it is bleedable. Whether or not there is enough volume ahead of the piston to affect good braking I cannot say, except that, possibly not, cuz if there was then you would not be having this problem.
Another possibility is that a pocket of air exists in the front brake system.Air is compressible,so some of the displaced fluid will be used up to compress the pocket and not leaving enough fluid to properly apply the calipers. You are gonna have to hunt for this pocket and get the front system working 100% first.If you spend a buncha time on the rear system, but can never get the fronts, well, that would be a total waste of time.Perhaps this is why the guys are talking about the bigbore M/Cs. The 1 1/32 chamber, at the same stroke, will displace 12% more fluid. To my way of thinking this is a design flaw(.Edit: my thinking has changed explanation below) I hope you have teflon hoses up front.They are a much stiffer, more flex-resistant design. This will ensure a better caliper application.
So that's the front mechanical half.
Now lets say the rear system was working perfectly. In this case the only difference to the front system is that the mini pushrod is replaced with brake fluid, and the front piston is hydraulically locked to the rear piston. If the rear brake system fails, the rear-most piston pushes the hydraulic fluid out and the mini pushrod is again called on to stop the car as in the previous description.
Once the fronts are working with the mini pushrod, you will have to bleed the M/C , so that the rear chamber is air free, before you can proceed to the rest of the rear brakes.
Now getting back to the fronts brakes;
I would really like to know how much reserve stroke that system has. And you should want to know as well.Here is how to figure that out; Have a helper jam on the pedal really hard, two feet hard. Then crack a bleeder.The pedal will, of course, drop.The amount of drop is the reserve. I would also capture the fluid expelled from the bleeder and measure it.
This volume represents the reserve. If you get just 1 or 2 cc,and, if you are absolutely sure there is no air in the system,then the system is less than marginal, allowing almost no room for error; as in hose flex,pad flatness, or caliper flex..etc. I would either move back to the 1 1/32 M/C and live with the pedal feel or abandon the system entirely.
But if you get something like 4cc or more then that is pretty good. Between 2 and 3cc is probably marginal, get the teflon hoses.
Now on a personal note I would not use that M/C at all.(Edit: thinking changed, see below). IMO, The front brakes are on the wrong end. If the big chamber is on the back, and the fronts are plumbed to it, then you don't have all these limitations. The first piston in being connected to the pushrod and pedal, means you can suck the piston back to it's parking spot with your toe.They are mechanically linked. The pedal travel is no longer a limiting factor. And forget about reserve capacity, there is plenty.A fluid loss in either front or rear system, simply leads to a lowered pedal. Yeah its scary when it happens in traffic, especially in a drum rear car, with poorly adjusted shoes, and a front hose pukes! Whooooee, I was a pumping!
So to recap; get the front brakes to 100% first.This is priority one.
PS, If you care to, you could disassemble that M/C, and inspect the frontmost piston. It may have a mini pushrod on the frontmost end. It is there to limit the piston travel into the grungy part of the casting. With careful observation and measuring it may be possible to shorten that stump some. A quarter inch in a 15/16 bore is about 2.8cc.So if you need the reserve there it is. But if your pedal is already less than about 7/4 or 1.75 from the floor, forget about it cuz cutting off the quarter inch will put the pedal ;whatever ratio it is (often around 7:1),times the 1/4 inch closer to the floor. Yeah I'm not sure that's real clear.
Ohhhhhhh, now I get it! They put the front brakes into the front chamber, cuz it gets the full 15/16 bore, duh. The rearmost chamber has the inter-works in it reducing the total piston area by the amount of the interworks. That is why the 1 1/32 M/C requirement for the factory type M/Cs. Cuz the net of those might only be similar to the 15/16 that you have.
Okay So I owe everybody I ever mightof ticked of as to M/C selection, a great big I'm sorry. See I learned something new. And I take back what I said earlier about not using this M/C. Clearly it is a good design, even if I don't like the loss of mechanical piston retrieval. I might even change mine, to see if I like it..
Ok that's it for now
Here's the deal;
Inside your M/C are two pistons linked but operating independently.If you were to disconnect the line to the rear brakes, and then stomp the brakes, this is what goes on; The footpedal is connected to the pushrod, which is connected to the rearmost piston. the end. Pedal goes down, piston moves forward. On the front of that rearmost piston is a Mini pushrod. When the rear piston moves forward the mini pushrod eventually reaches the front piston. If you have enough pedal travel left, the front piston will push fluid out to the front calipers. if you still have pedal travel left the brakes will apply and the pressure will build, stopping the car.
So the two most important things are sufficient pedal travel, and the front piston returning to its rear-most parking spot. Since SSBC in there infinite wisdom swapped the working chambers, rear to front, there is now no mechanical way of pulling the front-brake piston to it's rearmost parking spot.So how are they returning the piston? ima gonna assume there's a spring in there, cuz seal retraction is not going to do it.Moot point cuz we already know it's returning by the fact that it is bleedable. Whether or not there is enough volume ahead of the piston to affect good braking I cannot say, except that, possibly not, cuz if there was then you would not be having this problem.
Another possibility is that a pocket of air exists in the front brake system.Air is compressible,so some of the displaced fluid will be used up to compress the pocket and not leaving enough fluid to properly apply the calipers. You are gonna have to hunt for this pocket and get the front system working 100% first.If you spend a buncha time on the rear system, but can never get the fronts, well, that would be a total waste of time.Perhaps this is why the guys are talking about the bigbore M/Cs. The 1 1/32 chamber, at the same stroke, will displace 12% more fluid. To my way of thinking this is a design flaw(.Edit: my thinking has changed explanation below) I hope you have teflon hoses up front.They are a much stiffer, more flex-resistant design. This will ensure a better caliper application.
So that's the front mechanical half.
Now lets say the rear system was working perfectly. In this case the only difference to the front system is that the mini pushrod is replaced with brake fluid, and the front piston is hydraulically locked to the rear piston. If the rear brake system fails, the rear-most piston pushes the hydraulic fluid out and the mini pushrod is again called on to stop the car as in the previous description.
Once the fronts are working with the mini pushrod, you will have to bleed the M/C , so that the rear chamber is air free, before you can proceed to the rest of the rear brakes.
Now getting back to the fronts brakes;
I would really like to know how much reserve stroke that system has. And you should want to know as well.Here is how to figure that out; Have a helper jam on the pedal really hard, two feet hard. Then crack a bleeder.The pedal will, of course, drop.The amount of drop is the reserve. I would also capture the fluid expelled from the bleeder and measure it.
This volume represents the reserve. If you get just 1 or 2 cc,and, if you are absolutely sure there is no air in the system,then the system is less than marginal, allowing almost no room for error; as in hose flex,pad flatness, or caliper flex..etc. I would either move back to the 1 1/32 M/C and live with the pedal feel or abandon the system entirely.
But if you get something like 4cc or more then that is pretty good. Between 2 and 3cc is probably marginal, get the teflon hoses.
Now on a personal note I would not use that M/C at all.(Edit: thinking changed, see below). IMO, The front brakes are on the wrong end. If the big chamber is on the back, and the fronts are plumbed to it, then you don't have all these limitations. The first piston in being connected to the pushrod and pedal, means you can suck the piston back to it's parking spot with your toe.They are mechanically linked. The pedal travel is no longer a limiting factor. And forget about reserve capacity, there is plenty.A fluid loss in either front or rear system, simply leads to a lowered pedal. Yeah its scary when it happens in traffic, especially in a drum rear car, with poorly adjusted shoes, and a front hose pukes! Whooooee, I was a pumping!
So to recap; get the front brakes to 100% first.This is priority one.
PS, If you care to, you could disassemble that M/C, and inspect the frontmost piston. It may have a mini pushrod on the frontmost end. It is there to limit the piston travel into the grungy part of the casting. With careful observation and measuring it may be possible to shorten that stump some. A quarter inch in a 15/16 bore is about 2.8cc.So if you need the reserve there it is. But if your pedal is already less than about 7/4 or 1.75 from the floor, forget about it cuz cutting off the quarter inch will put the pedal ;whatever ratio it is (often around 7:1),times the 1/4 inch closer to the floor. Yeah I'm not sure that's real clear.
Ohhhhhhh, now I get it! They put the front brakes into the front chamber, cuz it gets the full 15/16 bore, duh. The rearmost chamber has the inter-works in it reducing the total piston area by the amount of the interworks. That is why the 1 1/32 M/C requirement for the factory type M/Cs. Cuz the net of those might only be similar to the 15/16 that you have.
Okay So I owe everybody I ever mightof ticked of as to M/C selection, a great big I'm sorry. See I learned something new. And I take back what I said earlier about not using this M/C. Clearly it is a good design, even if I don't like the loss of mechanical piston retrieval. I might even change mine, to see if I like it..
Ok that's it for now
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