15/16" bore manual master too much for power C5 brakes

**yody** · 10-11-2005

Finally got front C5 brakes and LT1 rear brakes on a 68 firebird that I am working on. It came with a 15/16" bore manual master cylinder, he is using a powerbooster. I modified the pushrod to be longer since the stock one was too short. Took awhile to get the brakes 100% bled but now there is no air coming out. The pedal still wants to go 3/4 of the way down to the floor, however the brakes work good. THe pedal is super sensitive and very hard to modulate. I am thinking that the size of the bore is too small and creating a low sensitive pedal. 1" bore seems to be pretty popular and works good on my car, however will a 1/16" differnece in bore size make that much of a noticable difference? thanks

**chicane67** · 10-11-2005

Well, I have the ID of a C5 cylinder at 1.36".

A 1.0" or 1.125" would be acceptable considering that your pedal is too sensitive and that its not moving enough fluid.... shown by the 3/4 pedal travel. Noticable difference..... ??..... yup. The sensitivity and pedal travel should equalize with a diameter change.

I myself would lean towards the disc/disc 1978 WS6 master cylinder (1.125").

**yody** · 10-11-2005

hmmm, on my car I have an LS1 cylinder, I was always told it was a 1" master?? maybe its not?

**chicane67** · 10-11-2005

An LS1 cylinder off of what ??

**yody** · 10-11-2005

off of an 98-02 firebird with LS1

**chicane67** · 10-11-2005

Its probably correct..... the number I am looking at is for a Y-body and they have a different assist unit.

But the above still stands for the sizing recommendation.

**yody** · 10-11-2005

any cast iron master you could recommend? don't want to mess with the fittings.

**chicane67** · 10-11-2005

Ok, here ya go

Originally Posted by chicane67

I myself would lean towards the disc/disc 1978 WS6 master cylinder (1.125").

**Noel67** · 10-12-2005

Yody,
I recommend that the 1" be used in the power mode. I have a 1" Corvette master that I can send Fernando if you guys want it. The 15/16" master works very well in the manual mode. Excellent modulation and the pedal effort is only slightly more than a boosted system. It does have a hint of sponge, but that is one of the trade offs for running a manual system. I'm running our hybrid and rear disc system on my daily driven 67 with the 15/16" in the manual mode. I have 25,000 miles on my brakes now and I wouldn't have it any other way. I wish you would have at least tried the system in the manual mode, I think you would have been surprised. Either way, I will call you Fernando today and arrange for the shipment of the 1" master.
During our development, both Jonathan and I logged several thousand miles each using the 1" and 15/16" master cylinders and then decided that the 15/16" was the ticket for manual. We never tested a power sytem since neither of our cars have enough vacuum.
My hydraulics catalog has the LS1 master listed as a 1" bore.

I didn't realize I was talking to the famous Yody this weekend. It was good talking to you. Very nice car by the way.

Noel Garcia
www.touring-classics.com

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**70 LS1** · 10-15-2005

I am pretty sure the C5 master is closer to 3/4" than 1". I tried using a 1" old school corvette master with my C5 brakes and they sucked. Worse than stock performance. I swapped in a C5 master and the brakes are much better than stock. I also compared the two next to each other and the C5 is quite a bit smaller than the 1" master.

**yody** · 10-15-2005

no way corvette is 3/4". Also thanks for trying but none of that will really help as you are just guessing.

Can you explain how your brakes "sucked"?

**70 LS1** · 10-15-2005

Sure. The pedal was extremely firm and difficult to push. The C5 brakes have a smaller piston in the caliper than the old C3 brakes. This causes you to push harder on the pedal to get the same clamping force on the rotors. I know for a fact I was using a 1" master, before. After talking with some C5 racers I got dimensions for the C5 master from 3/4 to 7/8 and determined that the C5 master would be the way to go. It does indeed have a smaller bore than the 1" corvette master I was using. It may be 7/8" vice 3/4", but either way it is smaller. The smaller bore creates a higher line pressure for a given pedal force. The higher line pressure in the C5 allows for better performance from smaller, lighter calipers.

Another thing you can do to adjust the sensitivity of your pedal is to alter the pedal ratio. By lowering the attachment point on the pedal you can lower the leverage assist built into it.

**70 LS1** · 10-15-2005

OK, just measured my C5 master and it is between 13/16 and 7/8", probably metric so it didn't come out exact.

**yody** · 10-15-2005

hmmm, I always thought you had to take out the piston to measure the correct bore? much better btw!! he already had his attachment in the lower hole

**70 LS1** · 10-15-2005

Think of it this way. The piston has to slide in from the rear, so it can't be any larger than the opening. The opening is 13/16-7/8". The bore is the diameter of the hole drilled through the master and the piston has to be within pretty close tolerances to ensure no leaks, even with seals. Don't forget you are putting over 1000 psi on the other side of that piston.

**yody** · 10-15-2005

yeah thats true, i guess its only the snap ring that holds it in, interesting. But then why would a 15/16 cylinder be so mushy

**70 LS1** · 10-15-2005

Could be the pedal ratio or maybe the power booster is giving too much assist. What are you using for a booster?

**427v8** · 10-16-2005

Pedal ratio wiould make a big difference too.

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15/16" bore manual master too much for power C5 brakes