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LBC m/c question

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Westfield_XI

Westfield_XI

Jedi Warrior
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I have a stock single port Spridget m/c on my kit car and it is undersized for the updated brakes I am running. I have Toyota 4-piston front calipers and MGB GT wheel cylinders on the rear drums and the Sprite m/c just cannot move enough fluid.

Does anyone know of a larger bore m/c that will fit into a Sprite pedal box? Maybe MGB or big Healy or Jaguar? If necessary, I will replumb the brakes for a dual port type if that is what is needed and I can of course alter the mount holes to fit.
 
"not moving enough fluid"??
If that is the problem then you don't have the brakes adjusted properly-
Do you mean that you need move "power"? If so then a bigger bore is the opposite of what you need.
Bill
 
I think he is saying that there is not enough fluid displacement for the movement of the calipers and wheel cylinders.
 
This could be an interesting problem, dimensioning brake hydraulics from scratch. I'm afraid you don't really want to "just stick something on."

But I guess you could start with the m/c diameter from the Toyota that had those calipers. If they were available on several cars, try the lightest weight car.

Then estimate from the fact that your car is lighter and increase m/c bore area (decrease brake force) proportionally by the percent of your cars weight to that of the Toyota.

When that size is obtained search catalogs until a m/c is found with a flange that might fit on your pedal box. Then modify the mounting to fit.

Rear brake sizing will probably have to wait until the car is driving. Adjustable pressure limiter valves do wonders. Testing is the problem. I have a fool-proof method, but it takes a test track.
 
You could go to a larger diameter master if you changed the brake lever accordingly. If you have changed the brakes themselve's that much you may be able to get by with lower pressure on the system so maybe just going to the larger master would work. It would be interesting to look over the system that activated the front calipers in the toyota but I'll bet it was power assist so you really can't go by it. Looks like a trial and error game to me. You may want to go to a Tilton or similiar system so that you can balance front to rear easily anyway.

Kurt.
 
You have it backwards. Increasing bore diameter is going to increase pedal force. For example, with the disc brake setup using the stock master cylinder, going from the 7/8" master cylinder on a Bugeye to a 3/4" bore master cylinder, reduces pedal effort by about 1/3rd. (not to be confused with better braking) A booster can be used to overcome a heavy pedal. (also not to be confused with improved braking)

I'm not sure you can "engineer" your brake system by mixing/matching brake components for size. I think it's going to take some real engineering to make it right.
 
Gerard said:
You have it backwards. Increasing bore diameter is going to increase pedal force. For example, with the disc brake setup using the stock master cylinder, going from the 7/8" master cylinder on a Bugeye to a 3/4" bore master cylinder, reduces pedal effort by about 1/3rd. (not to be confused with better braking) A booster can be used to overcome a heavy pedal. (also not to be confused with improved braking)

I'm aware of the engineering(I think), Gerard, but his problem seems to be volume. If he has greatly increased the volume of the brake cylinder's then he should be able to increase the volume of the master with no ill effect. Yes it will require more pedal pressure but he may be able to live with that. My concern would be brake balance so if it were me I would be getting ahold of Tilton.
[BTW mechanical engineer though thats not what I do]

Kurt.
Click to expand...
 
Gerard said:
You have it backwards. Increasing bore diameter is going to increase pedal force. For example, with the disc brake setup using the stock master cylinder, going from the 7/8" master cylinder on a Bugeye to a 3/4" bore master cylinder, reduces pedal effort by about 1/3rd. (not to be confused with better braking) A booster can be used to overcome a heavy pedal. (also not to be confused with improved braking)

I'm aware of the engineering on this, Gerard, but if he has greatly increased the volume of the brake cylinder's he should be able to increase the volume of the master with no ill effect. He could change the pivot of the pedal but hen he would decrease the voume while reducing the force on the pedal.Nothing gained. Yes,increasing the size of the master will increase pedal pressure but his main problem seems to be volume. He may be able to live with more pedal pressure.

If it were me I would be concerned with brake balance and would be getting ahold of Tilton for ideas.
[BTW mechanical engineering degree here]

Kurt.
Click to expand...
 
Welll--did'nt appear that my message had gotten thru!! Guess inthe wild's fo So Dak I'm lucky to have any connection!

Kurt.
 
Nomad is on the right track of things. Carroll Smith has a good writeup on brake systems in one of his books. I agree it sounds like a fluid volume issue. If the car is fitted with a 1967ish type metal frame mounting pan and uses separate cylinders with integral reservoirs for the master and the clutch, it should not be too much of a problem to find a larger master that pushes more volume into the braking system. May have to use a remote reservoir, but this is not much of a problem. I can definitely see an issue with the 4-pot Toyota calipers needing more fluid movement. Pedal effort will increase, but not so much to be a problem in this light of a vehicle. Pedal force could be changed by changing the leverage ratio of the pedal. Maybe a MGA/MGB pedal/pedal box could be used with a longer length below the pivot.

Virtually all the master cylinders used on racing sports cars have the same mounting configuration and the same pedal stroke limits. These cylinders all move about 1.10". The cylinders used on the formed metal mounting pans in the later Spridgets have an angled mounting whereas the racing cylinders are usually straight up and down. This can be overcome with a simple adapter to bolt between the two. Aftermarket cylinders from AP/Girling/Wilwood/Tilton can be had with bores as large as 1.125". The volume will go up with the square of the difference in radius.

Check out the various websites for brake systems for racers (Tilton/Porterfield/CV Products/Central High Perf./Wilwood/Afco/etc.) and you will see all sorts of potential cylinder applications.

IIRC, Girling made some remove brake booster systems back in the 60's or maybe 70's. If pedal effort becomes such an onerous condition, something like that could be used, but I doubt it would.

Some additional thoughts to consider.

HTH, Mike Miller
 
I understood the issue was about volume. I was only commenting that the concept of reduced pedal effort was backwards.

Looks like a good brake tech link aero... I'll have to spend some rime checking that out.
 
Thanks for all the information, I am going to mull this over for a while before I figure out which way to go.

So far it sounds like I have a 7/8" bore m/c. And assuming that I need to move a greater than stock amount of fluid, since I have larger bores on both front and rear, to get sufficient pad/shoe movement, I will be looking around for a cheap 1" or 1 1/8" m/c. If I can find a suitable part that will mount up in a similar fashion, I will give that a try. Any suggestions offhand?
 
Gerard said:
I understood the issue was about volume. I was only commenting that the concept of reduced pedal effort was backwards.

I think we're in complete agreement, Gerard, I probably did'nt word it correctly. Larger master equal pedal pressure gives lower system pressure. Equal system pressure with larger master equal's more pedal pressure. larger master will increase volume but from your TR article, not much more volume should be required for the fronts. Still I would be mainly concerned with balance. It sounds like Westfield has a heck of a fun car but you don't want it to swap ends when you get on the brakes hard nor do you want it to not realize its complete potential because of unbalanced brakes. I would be checking out Mikes sites.

Kurt.
Click to expand...
 
Westfield_XI said:
Thanks for all the information, I am going to mull this over for a while before I figure out which way to go.

So far it sounds like I have a 7/8" bore m/c. And assuming that I need to move a greater than stock amount of fluid, since I have larger bores on both front and rear, to get sufficient pad/shoe movement, I will be looking around for a cheap 1" or 1 1/8" m/c. If I can find a suitable part that will mount up in a similar fashion, I will give that a try. Any suggestions offhand?
Click to expand...

I believe that the 67 master is probably a 3/4 inch bore. Least that is what was fitted to the mk3 sprite and mk2 midget just before yours with the same brakes. As mentioned compare cylinder area's when makeing decision's not diameter's

Kurt
 
nomad said:
Westfield_XI said:
Thanks for all the information, I am going to mull this over for a while before I figure out which way to go.

So far it sounds like I have a 7/8" bore m/c. And assuming that I need to move a greater than stock amount of fluid, since I have larger bores on both front and rear, to get sufficient pad/shoe movement, I will be looking around for a cheap 1" or 1 1/8" m/c. If I can find a suitable part that will mount up in a similar fashion, I will give that a try. Any suggestions offhand?
Click to expand...

I believe that the 67 master is probably a 3/4 inch bore. Least that is what was fitted to the mk3 sprite and mk2 midget just before yours with the same brakes. As mentioned compare cylinder area's when makeing decision's not diameter's

Kurt
Click to expand...

Kurt, isn't there some sort of relationship between the dia and the area? Given that the stroke would be similar in length to the one I have now, going up in dia would give a larger area and also a larger volume IMHO.

It looks like a Tilton m/c would be an easy bolt-on and is available in 1" and 1 1/8" for only $85 online as a kit with various reservoir options included. I am going to try a 1" kit and see what happens to the feel. if it doesn't work out, I will try the 1 1/8" one and put the other up for sale.

Great link to the Toyota brake install info. I wish I knew about that when I did mine, I would have tried to use the vented discs and wider calipers. As it is, I have a set of Xdrilled Spridget rotors installed now.

Can anyone educate me on residual pressure valves as used in single circuit brake systems? As I understand it, using a 3 to 5 psi valve will keep the pads closer to the rotors without causing a dragging brake if used on a disc system. Or will keep the shoes closer to the drums on a drum system, is this correct? In my mind I think of them as serving a similar purpose to giving the pedal a quick pump when approaching a curve, is this an accurate analogy?
 
Westfield_XI said:
Can anyone educate me on residual pressure valves as used in single circuit brake systems? As I understand it, using a 3 to 5 psi valve will keep the pads closer to the rotors without causing a dragging brake if used on a disc system. Or will keep the shoes closer to the drums on a drum system, is this correct? In my mind I think of them as serving a similar purpose to giving the pedal a quick pump when approaching a curve, is this an accurate analogy?
Click to expand...

If you want to use in-line residual valves... 2 PSI for front disc, 10 PSI for rear drum. This is what I do with Morris Minor Disc brake conversions - works great. I use Wilwood residual valves.

If you order a Tilton, I believe they come both with or without internal residual valves, so make sure you get the right one.
 
Thanks, Gerard. One question, though. Are the valves installed near each caliper or can I just put one in the line leading to the tee that splits the fluid between the front calipers and then another in the line to the rear drums. In other words, do I need two valves or four?
 
Master cylinder volume is figured using the radius squared times pi times the length of the stroke. Increases would be figured as the difference in the
two radius's squared calculated as a percentage would give you a general idea in what to expect for pedal pressure. If I do the math right an increase from 3/4 master to 1 inch master will require about 1 3/4 more pressure on the pedal for the same system pressure. You will gain the same in volume of fluid moved. If I'm not thinking of this right I'm sure some helpful members on this forum will pipe up!

Best of luck

Kurt.
 
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