What Master Cylinder Size for FV

[sgkarter4]

Need some advice buying all new master cylinders for a Vee. Stock drum brakes and a hydraulic clutch assembly. I wish I knew what the clutch cylinder was but I do not so advice for that may not be possible. Advice on the front and rear master cylinders would be great though. Thank you in advance.

[pacratt]

5/8" for fronts.
3/4" for rears.
3/4" for clutch. (assuming you have a Tilton or CNC or similar clutch cylinder)
Bias adjuster at the brake pedal a must.

Just my $0.02 worth.
Glenn Lange

[Matt Clark]

this is part of an email Jim Schings from SR told me when I was looking at new parts...

"With bore size, there are no free lunches. A larger bore will give you less pedal movement. (good), but less brake pressure at the caliper. (bad) A smaller bore gives you more pressure (less effort to stop), but more pedal movement.
Almost ALL Vees use a 5/8 bore on the fronts and USUALLY a 5/8 on the rear. (a 3/4 on the clutch). Some people have used a 3/4 on the rears, since you want less braking pressure at the rear. However the VW slave cylinder front and rear bores are already different sizes, so a 5/8 master should be used on both front and rear.
The VW slaves cylinders provide about a 60/40 front rear ratio. You really want more like 70/30 for racing. (That is why you use the adjustable bias.)"

but as always, opinions & experience vary.

[budawe]

A minor correction, the size of one of the bores in a hydraulic system does not effect psi. If you have x psi on one end you'll have same psi on the other. A larger master will displace more fluid and that will translate to more movement of the wheel cylinder, but at a cost of more pedal pressure, not psi pressure, foot pressure, to effect the same psi at the wheel cylinder.

Using a larger master will help if your brakes are too sensitive and lock up easily. Many folks with the new Carbotechs are going to larger masters for this purpose. I recommend 5/8" masters on both ends for our wheel cylinder sizes. The smaller master will give a lighter feel to the pedal.

Do not use a larger master on the rear or you'll find setting the bias more difficult since the rear wheel cylinders will move faster than the fronts and that's exactly what you don't want.

[Daryl DeArman]

A minor correction, the size of one of the bores in a hydraulic system does not effect psi. If you have x psi on one end you'll have same psi on the other. A larger master will displace more fluid and that will translate to more movement of the wheel cylinder, but at a cost of more pedal pressure, not psi pressure, foot pressure, to effect the same psi at the wheel cylinder.

Yes on the same psi at both ends of a hydraulic system.

However, IF you decrease only the MC bore size and nothing else the PSI in the system goes up.

[Steve Davis]

and nothing else[/U] the PSI in the system goes up.

*FOR* the same pedal pressure.

[Daryl DeArman]

*FOR* the same pedal pressure.

Like I said "change nothing else", which would include pedal pressure as well as pedal geometry/ratio.

We are talking about the effect of changing one variable, the MC size.

Smaller MC = more PSI given no other changes.

[pacratt]

5/8" for fronts.
3/4" for rears.
3/4" for clutch. (assuming you have a Tilton or CNC or similar clutch cylinder)
Bias adjuster at the brake pedal a must.

Just my $0.02 worth.
Glenn Lange

When I made the above statement, I was assuming the user was going with everything stock, including shoes. This is just what I've always used without any complications.
But, yes, Carbo shoes and your driving (braking) style could require a different set-up.
sgkarter4, read EVERYTHING that's coming in here... it's great technical info from a lot of long-time Vee "tinkerers".

Glenn Lange

AND... forgot to mention that line size can also play a roll. Most Vees I know of use 3/16" hard line & -3AN for the whips.

[cjohns90]

Gents...first time FV owner & first time poster...just picked up a 1968 Race Craft Engineering FV...80% complete, just putting everything to spec...I need to rebuild madters too, currently has a duel brake MC 1" & 11/16" and 7/8 clutch...based on the above, this seems way out of whack?

Drums/shoes stock, hydro clutch slave (buggy style?)

I cant find a replacement 1"-11/16" anywhere...should I be changing to a single duel 5/8" brake & 3/4" clutch or fab up 3 MC rig 5/8, 5/8, 3/4?

Your help, wisdom & advice is sincerely respected!

Christian

[Dietmar]

Christian:
You are correct- these are way out of whack

Most use a 5/8 master front and rear with a bias bar between the two and a 3/4 clutch.

Dietmar
www.quixoteracing.com

[smsazzy]

Christian:
You are correct- these are way out of whack

Most use a 5/8 master front and rear with a bias bar between the two and a 3/4 clutch.

Dietmar
www.quixoteracing.com

I will add that if you are using the new XP8 compound from Carbotech, I would recommend to go to .70 or 3/4" on front and rear masters. But then you are pretty much committed to the XP8's as any other shoe will require too much pressure in a Vee.

Traditionally, we run 5/8" masters to reduce needed pedal pressure. The new XP8 compound from CT requires far less pedal pressure and 5/8" masters result in a lot of lock ups.