Have I got the wrong size master cylinders?

As title states i'm after some advice on the correct size master cylinders. I have a bias pedal box with 7/8 master cylinder for the front (running big brake kit off ebay) which consists of Ve commodore calipers and .625 rear master cylinder running ed falcon rear calipers. My problem is it always locks the rears first. Even with a wilwood proportioning valve in, it still just locks up the rears first at its lowest setting. Anybody come across this before? how do I fix it? cheers Kieran

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Originally posted by fordsy

Swap them around and try again.

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have tried a .625 (5/8) on the front and had no luck.

No I haven't. I have been told that the front will require the bigger master cylinder as its twin piston where as the rears are single. is this right or am I been led up the garden path Steve? cheers

If you have enough info on your car, you can always ask Tilton their recommendations about mcyl sizes.

The link is here. http://www.tiltonracing.com/content.php?page=faq&view=9

For a given area a smaller cylinder will give more pressure from the same amount of travel than a larger cylinder.

The only problem with a smaller cylinder is that it displaces less fluid than the larger one and you can run out of travel without getting full pressure into the brakes.

Try the 5/8 on the front and the 7/8 on the rear and see if it's any better.

Dont feel too bad. I've done flexy and I thought it was a PITA too.

Ash

Kizza

As Fordsy has already stated - it's all about hydraulic ratio. A smaller M/C has the ability to apply a larger pressure for a given push which is transferred to the brake calliper pistons.


Crunching the numbers for the 7/8 master with a push on the pedal of 100lb (ignoring pedal ratio).

Area of the piston (0.875)^2*3.14 = 2.404 sq inches
Pressure 100/2.404 = 41.59psi line pressure.

Using a 5/8 (0.625) master cylinder

Area of piston (0.625)^2*3.14 = 1.227 sq inches
Pressure 100/1.227 = 81.53 psi line pressure.

Hope this helps demonstrate why the smaller MC gives the greater line pressures available to the brake calliper. Assuming the same brake calliper is used in both of the above scenarios then the smaller MC will give the better braking effort.

The pedal stroke will also be greater as both the 7/8 and the 5/8 cylinders will need to move the same volume of fluid to operate the brake calliper piston. This is where pedal ratio comes into play to also boost the line pressure by effectively multiplying the force applied to the MC piston.

With a pedal ratio of 6 : 1 the force applied to the MC will be 600lbs - which effectively multiplies the line pressure by the same factor.

A compromise needs to be made between pedal ratio and the required sweep of the pedal to acieve the full stroke of the master cylinder. Make the pedal too long (to get better ratio) and it will hit the floor before the MC has reached it's limit. This can also happen if the MC is too small and cannot supply the calliper with sufficient fluid to operate correctly.

Just a follow up to anyone who is interested. Tried the swapping master cylinders theory and no go. Had way to much travel, this indicated the 5/8 master didn't have suffient volume for the ebay brake kit. I was already confident this was going to be the case, however it cost nothing so thought i'd give it a go. So ended up taking Rhys' advice and went with .700 for the fronts (ebay kit) and .750 for the rears (falcon). This resulted in excellent brakes and a slightly hard pedal but not too hard. The bias is just abotu spot on and with some adjustment will be 100%. Just thought i'd throw this info up incase anyone wants it later down the track. Again cheers to all for your help.