Brake Master Cylinder Sizing

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subrew
Posts: 32
Joined: September 13th, 2007, 12:23 am

Brake Master Cylinder Sizing

Post by subrew »

Hello,

First off, yes I searched, and I found a thread with a similar title that Keith started. But I had additional questions.

One of the master cylinders on my AD has sprung a nice leak, and the other has issues with galled up threads on the brake pipe connection. I have been fighting small seeps, and the bad cylinder has suddenly turned into a puddle maker overnight. So, I'm just going to replace both and have that job checked off.

From what I can pull off the Girling cylinders, I have two 5/8" cylinders with a fairly basic balance bar with forward bias. The car suffers from a longish pedal. It stops just fine, but it seems to take a lot of travel to really get the bite. I have the drum adjusters set correctly, and there is no significant free play in the pedal assembly or connections to the master cylinders. Like I said, the overall braking power is very good, and the balance is spot on, it just takes more pedal movement than I would want. Under race conditions, everything performed great at Thunderhill two weeks ago, except of course for the minor seeping previously mentioned. And now the puddle...

So...get new 5/8" master cylinders and hope my longish pedal is caused by the leaky cylinder? Or would getting 3/4" master cylinders help with the longish pedal?

I want proper braking, but would also enjoy shortening the pedal travel a wee bit, as that would allow me to run the steering quickener that otherwise got in the way of the back of the brake pedal arm. Can i have my cake and eat it to?

Thank you,

Chris H.
'64 AD
http://www.subrew.com/tmp/graphics1.jpg
smsazzy
Posts: 703
Joined: June 24th, 2006, 5:56 pm

Re: Brake Master Cylinder Sizing

Post by smsazzy »

Go with the 5/8 cylinders.

How many clicks are you backed off on your brake adjusters?
Stephen Saslow
FV 09 NWR
cendiv37
Posts: 386
Joined: June 25th, 2006, 7:29 pm

Re: Brake Master Cylinder Sizing

Post by cendiv37 »

There are lots of options, depending how much you are willing to change. Remember, if you increase the master cyl. diameter, it will require higher pedal pressures for the same braking force.

I redesigned my brake pedal for more leverage AND went to 5/8 x 5/8 cylinders to get the pedal effort where I wanted it. However, I was ONLY able to do this after I went to the Carbotech shoes in front. The shoe wear is considerably less with these than with white box "stock" shoes increase in pedal during a race is reduced (it's almost all at the front in either case). Another part of the pedal redesign was thinning it at the top where it would otherwise interfere with the pitman arm. The bending forces on the pedal are smallest at the top. Do not thin it near the master pushrods! My pedal is probably overkill, but I really don't want it to break!

If you change both masters from 5/8 to 3/4 , your pedal effort will go up by about 45% for the same stopping power (proportional to the increase in area of the cylinders). Your pedal travel will go down by about 30% for the same shoe movement. The bias adjustment should not be effected.

If you change only the rear master to 3/4, you can come up with something in between. I've done it. It works just fine. In this case, the pedal effort will go up by about 18% and the pedal travel down by about 18%. You will have to move the bias adjustment towards the rear to make up for the cylinder size differences. For example, if you now have 60% bias to the front, you will need about 50/50 with this set-up.

Make sure you make/leave clearance between the back of the pedal and the pitman arms and tie rods. You need this clearance at the end of the race, AFTER the pedal has dropped. Go gently here: I have personal experience with getting it wrong, and it wasn't just me that suffered the consequences :oops: .

Do what you can to minimize any "lost motion" in the pedal to cylinder push rod system. However, a big caveat: there MUST be some free play at the pistons in the master cylinders in order to guarantee that the pressure is released when you get off the brakes.

Lastly, the standard vented covers for our typical master cylinders (both plastic and metal) "breath" as you use the brakes. Apparently in our application, there is brake fluid splashed into the breather path and it come out the vent as we run. I always found some brake fluid in the nose of my car after each session UNTIL I got some machined brake covers with bellows type vents/seals (like EVERY American car has had since the 1960's!!!) that allow the air to go in as the fluid goes down, but keeps the fluid from seeping out due to the "breathing". These also keep moisture out of the brake fluid (which is why they are used on EVERY American car since the 1960's!!!) which makes the fluid last almost forever. I bought these covers years ago from Lybarger. I don't know if he stocks them or not, but they are worth every penny.

my $.02
Bruce
cendiv37
subrew
Posts: 32
Joined: September 13th, 2007, 12:23 am

Re: Brake Master Cylinder Sizing

Post by subrew »

Thank you for the great response, this gives me a lot to chew on.

Chris H.
subrew
Posts: 32
Joined: September 13th, 2007, 12:23 am

Re: Brake Master Cylinder Sizing

Post by subrew »

Jut a quick follow up:

Got two new 5/8" cylinders installed this past weekend, fresh fluid, everything bled and wheel cylinders adjusted. The results are less pedal travel, and a much stiffer pedal than before. So it looks like a positive result.

I'll be running at Buttonwillow on Saturday, so we'll see if any issues crop up then.

Thanks again for the help.

Chris H.
'64 AD
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