Master cylinder bore size question

michael evans

Well-Known Member
My Power master was dying so I replaced it with a manual.

Wonder if I did the math wrong or what.

The system worked well with the Power master but cycled often and was starting to leak at the pump.

The set up is Wilwood four-piston disk in the front and 3/4 wheel cylinders with drum on the rear.

Calculations was telling me that I needed a 7/8 inch master to get the best brakes

I did change the pedal to the manual/vacuum style.

Well the brake are crap. They will stop the car but are not good for panic stops or even for shut downs at the track.

Should I have gone with a bigger bore master cylinder ?
 

TexasT

Texas, Where are you from
How does the pedal feel? Is it solid, or real spongy? Have the front hoses been replaced? What about the proportioning valve on the frame, has it been replaced?
 

Chuck Leeper

Toxic old bastard
Staff member
Calculations was telling me that I needed a 7/8 inch master to get the best brakes
How were the calcs performed?
A 7/8" bore will produce significantly less volume, than a stock, 1"+ mc. However, the upside is more pressure is created using the same ratio and pedal pressure.
We use a brake pressure gauge, when troubleshooting a system.
Suggest you call Wilwood, and ask.
They told me that the 1 1/16" I had, would work w/ the HB, when I ditched the PM. They were right.
 

No disintegrations

Well-Known Member
The smaller master cyl piston gives more force like Chuck said at the cost of more distance/pedal travel. Yours may be bottoming out before you get to the right force. Is the pedal soft or hard?
 

michael evans

Well-Known Member
The pedal is hard and solid (no air in the system). I had someone else drive the car to get his opinion

Everything is almost brand new in the system from the master to the wheels.
I rebuilt the rears with the S-10 wheel cylinders (7/8 inch), new portion valve and new four- piston front brakes.
The system worked well with the PM


Seems like I either need to go with a one- inch master or a vacuum assist.
 

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michael evans

Well-Known Member
Anybody know off the top of their head what size the bore of a PM is ?

I have two out in the garage I can measure when I get out there.
 

Turbo6inKY

Short Guy
I'm going to echo Chuck and recommend you call Wilwood.

Wilwood recommended a 15/16" bore master for my Dynalite 6R equipped front brake kit with a manual pedal (Powermaster pedal, I think it's 6:1). I also got a brake pressure gauge and I'm at 1100psi front, 780psi rear after adjusting the prop valve to keep the rears from locking up.

Master cylinder bore is generally aligned with piston area. The caliper piston area determines how much fluid volume you need to generate a given pressure. Master cylinder piston travel then determines how much pressure is generated. Pedal effort and travel is manipulated with power assist and pedal ratio. All if it can be put into an equation and manipulated to get what you want. You can spend a few days learning the math and building the model, or you can just call Wilwood.
 

Turbo6inKY

Short Guy
Also, which pads came with your setup? My old Wilwood 2508 kit with the 10.75" rotors came with Wilwood Q compound pads, and they sucked. Your kit looks like the same hub with an 11" rotor?

The hydraulics might be working just fine. The pads could just not have the kind of cold bite you want. Look at EBC Yellowstuff or the Porterfield R4S compound. Lots of people like Hawk, and the HP+ is an amazing pad, but it grinds rotors to bits with a quickness. I found it was just too expensive to run, even with the 50% off coupons I'd win at events. Instead of two or three sets of pads per rotor change, it was two rotor sets per pad.

Here's a chart of the CF of some of Wilwoods street/strip pads:



You can see the Q compound needs some heat in it before it starts working. The BP-10 is far more consistent over a broader temp range.

Small changes in Cf have huge changes in feel and braking torque. The Q compound's cold bite is horrible.
 

earlbrown

runs with scissors
I'm amazed with all the ''you need XXX bore for this size calipers....'' stuff without even doing the math on the distance from the pedal fulcrum to the pivot point and the contact point.
 

michael evans

Well-Known Member
I ran a 7/8 bore master with wilwood 12.19" front brakes and stock drums with s10 cylinders. Worked perfect.

That is part of what has me baffled. The PM has a 7/8 inch bore and besides the pump leaks it worked well. With the manual conversion with a 7/8 inch bore it is not working so well.

Back to the drawing board.
 

earlbrown

runs with scissors
Of course not. you went from a 7/8 bore with assist to a 7/8 bore without assist.


the extra oomph have to come from somewhere. ....and that load comes from the front seat track mounts
 

michael evans

Well-Known Member
Of course not. you went from a 7/8 bore with assist to a 7/8 bore without assist.


the extra oomph have to come from somewhere. ....and that load comes from the front seat track mounts
I was hoping the difference with the different brake arms would make up for the assist.
Guess in my case I should have went up in bore diameter.
 

RmvBfrFlght

Well-Known Member
Smaller in bore size. That will increase the brake line pressure with the same foot power input. Better pads is also helpful, BP20 or the E pads.

RemoveBeforeFlight
 

michael evans

Well-Known Member
That was my thinking as well. Since the PM had a 7/8 bore as well, I could do that size in a manual as well.

I made the call to Wilwood and he told me since I changed the front calipers out to the bigger four-piston kit it needed a larger bore on the master cylinder.

The 7/8 inch has an area of 0.600 for the bore where the 15/16 inch has an area of 0.690. The power assist made up for the little difference.
 

Chuck Leeper

Toxic old bastard
Staff member
I've been thru all this :poop: , while using the same ft brakes on a 55 Chevy.
Pad changes, vac boost with dual 8", all resulted in poor performance.
The key to finding the issue is, as I've said before, is a brake psi ga. All the brake mfrs provide specs.
The use of the HB took the line psi from a miserable 8-900, to nearly 2000.
If you are dealing with <1200, u are pounding sand in a rat hole.
 
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