Master Cylinder Stock Sizes?

Hello,

I am having braking issues. I've narrowed it down to changing the bore size on my master cylinders. While I'm at it, I plan to replacing my clutch cylinder too, simply to make it a bit softer. So, before I begin, I have a few questions...

1. What is the bore size of our factory installed clutch and brake cylinders?
2. (for GT40 racers) Under racing conditions, what bore sizes did you find work best.
3. What brand cylinders do you recommend? (I'm thinking of going with AP)
4. What is the piston size on the factory installed Wilwood caliper?

Thanks for any help!

Randy
 
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Rick Muck- Mark IV

GT40s Sponsor
Supporter
Brake masters are Wilwoods, .625 and .700 for both the street cars and the "R" versions. Don't recall the caliper info offhand but I will look it up. I think the clutch is a .700 but will need to double check.

Dennis Olthoff uses the standard masters on his MK II race car so I suspect they work just fine.
 
Wilwood calipers are 1.75" pistons all round. Brake m.c. are 5/8" and clutch .700". I changed my clutch m.c. to 5/8" to reduce pedal effort.
 
You're probably right Rick. I only looked at one brake m.c. Returning to OP, what is your braking issue Randy?
 
The brake issue is... Let's just say, it's very, very scary coming into a corning at 150mph and having to pump the brakes a few times to get a solid pedal feel. I bled the brake several times thinking it was air. Some air did come out the first time, but nothing since. They have always felt soft and I just can't live with it anymore. It's way too dangerous.

I talked with Dennis Olthoff this morning and he recommends staying with the Wilwood brand and using .700 master cylinder on rear and .625 on the fronts.

I'm also going to change the clutch cylinder from .700 to .625 to soften it a little.

Is this the right combination?
 

flatchat(Chris)

Supporter
Sounds more like pad knock off -- make sure the rotors run true ( within .002" tir)-- either re machine or replace -- and if replacing prefer pre bedded rotors.

Tell us more about the rotor set up, pad compound, calipers, fluid etc
 
It's the factory Superformance Wilwood set-up. I'm also using the factory installed pad compound. I don't know what compound they are. They work fine after a few quick pumps. My GT40 brakes have always been mushy.

From what I've found in my research, is that because my GT40 is early production car that I have an older style or model master cylinders (?). That's what I've heard anyway. I know another early SPF GT40 owner replaced his master cylinders and now his brakes work fine. Besides, I only have 3000 miles on them.

So, I'm 99% sure it's the master cylinders. I just wanted to know what cylinder sizes the guys who race their GT40s are using... and the brand that works best for them.

Thanks.
 
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flatchat(Chris)

Supporter
Is the master cyls reservoirs above or below the caliper level ???
ie if the res is below the caliper line (floor mounted)then a drain back may happen.
Go do a physical on the rotors in situe and lets know what the indicater reading is for face run out
 

Rick Muck- Mark IV

GT40s Sponsor
Supporter
I have supplied replacement masters to a couple of early car owners. Don't know what or if there is an issue. No cars have had problems in the last few years I am aware of. In fact one owner is very impressed with his braking when he jumped on the pedal...."the hand of God came down and stopped me!" or something to that effect was his comment.
 

Jack Houpe

GT40s Supporter
The brake issue is... Let's just say, it's very, very scary coming into a corning at 150mph and having to pump the brakes a few times to get a solid pedal feel.
There's something wrong, find it fix it, you only get that chance to recover a few times before disaster happens. Thank God your quick to react.
 
I would follow Chris' advice first. More than about .004" disc runout will cause pads to retract. Check wheel bearings are tight. Assuming you have fresh 500 degree+ temp brake fluid? I'm surprised the standard Wilwood BP10 pads work for you. I get extreme pad fade after 2 laps on a track day. Tried BP20 pads, still fade. Went to Wilwood B compound race pads for track use. I have had 2 clutch m.c. fail, but no brake ones. I think it was due to degradation of rubber hose from reservoir to m.c. Fluid was discolored.
 
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Guys:

Keep in mind brake fluid absorbs water. Bleeding your brakes should be done before a race or a track day. I personally would bleed before each race if time allowed. If you feel you have boiled the fluid change it. You can buy brake temp paint kits and caliper temp strips to find out how much heat you are getting in the system. NASCAR teams have gone to running anti knock back springs behind the pistons in the calipers to combat knock back so the drivers do not have to pump the pedal to get the pads seated against the discs. This is done mainly at the road course events.
 
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I had the same symptoms you describe on my GT350 clone when I converted to rear discs. A wilwood 2psi (blue) residual pressure valve fixed it. I use the same valves on my wilwood-equipped Pantera and they work perfectly. FWIW....
 
Thanks for all your great comments.

A big Thank You to Dennis Olthoff for his track-proven advice!

Dave, good advice... I'm going to change the hoses too. There's a good chance that's part of the problem too.

I'll let you know...
 
if you have a floor mounted pedal assy, I would highly recommend a 2lb residual valve in both front and rear lines, I have the same problem with the F5000, valves are on order, I don't find it an issue, as habit has taught me to tap the pedal before I get to the mark, but owner has yet to learn.

cheers John
 
I had the same symptoms you describe on my GT350 clone when I converted to rear discs. A wilwood 2psi (blue) residual pressure valve fixed it. I use the same valves on my wilwood-equipped Pantera and they work perfectly. FWIW....

I forgot to mention, I drove a friend's Pantera which had the same Wilwood brake system as mine, but without the RPVs. A 30 minute drive on the freeway with no braking would result in enough pad knockback that when you came off the freeway, the pedal would almost go to the floor the first time you stepped on it. It was then perfect after that, but that is definitely not what you want.

Installing RPVs fixed his problem instantly.
 
The brakes are now perfect! I had the master cylinders replaced and modified the fluid reserves with braided lines.

Thanks again for everyones advice and a BIG THANK YOU to... Dennis Olthoff and Rick Glover!
 
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