Master Cylinder Size

I am upgrading the brakes on the car that I am building my dad. We are going with disk brakes on all four corners. The rotor size 12.19" with 4 piston calipers. I was planning on running a Wilwood master cylinder along with the brakes and was wondering what size of piston bore to buy. The have them in 7/8", 1", 1 1/8" diameters. I initially thought that I would go with the 1 1/8" diameter because it would provide the most fluid to the calipers. When I called Wilwoods tech line to double check what would work best with this set up they told me to use the 7/8" diameter master cylinder because it provides the most pressure and that is what I need with a manual brake set up. This goes completely against what I have been told by many others. What are your thoughts? And what are your experiences? Everyone that I talked to said they would go with a larger diameter master cylinder and a few had first hand experience saying that it improved the feel and the braking abilities on their cars.

Thanks for your feedback.


Piston Count: 4 Piston Area (In²): 4.80 Piston Type: Stainless Mount Type: Lug Brake Pad Plate: 7112 Brake Pad Area (In²): 6.36 Brake Pad Volume (In³): 2.1 Pad Compound: BP-10

 

A larger master cylinder piston translates to more physical movement of the caliper pistons. Power brake setups usually go higher in MC piston area to allow quick take up of pad to rotor clearance for a tighter pedal feel. A smaller MC piston provides more pressure to the piston but requires more pedal travel for the same caliper piston movement. Manual brake setups use a smaller MC piston for this reason. The 1 1/8" MC will have a very hard pedal and require more leg strength than the 7/8" piston assuming the same pedal lever length.

 

What size would recommend of the three?

 

I would run the 1" personally , the pedal maybe a little soft with the 7/8

 

For power brakes, go big. Manual, go smaller. I have a 1 1/8" bore on my power disc brakes and in the interest of science I have run just about every other kind of master cylinder I could think of before settling on this one. For manual brakes, you want smaller. The bigger the bore, the less pedal travel but the more pedal effort. Smaller bore means more travel for the same amount of fluid, but less effort.

 

I have manual brakes (and would NEVER go power in this small car!), and am using Wilwood 260-8555-P on mine (the P just means polished). I still have drums out back but the braking is firm, and it will stop on a dime.

It is a 1" bore.

 

I'm very pleased with my all stock manual front disk/rear drum brakes. Front disks, master cyl, and proportioning valve all came from a donor Maverick. They work great.

 

I have stock '74 front discs, Explorer rears, and use a MC from a '91 Ranger....15/16" bore diameter. Works flawlessly.

 

This quote from an article corroborates what others said including the Wilwood tech.

"MASTER CYLINDER BORE
To activate a braking system, a given volume of brake fluid has to be moved from the master cylinder to the calipers and wheel cylinders. Just like engine displacement master cylinder volume is equal to the bore times the stroke. If you use a narrow diameter master cylinder, you will need more stroke to achieve the necessary volume required to activate the brakes. The longer the stroke, the easier the pedal will push. Conversely, with a larger master cylinder bore, the stroke will be shorter. A large bore master cylinder requires more pedal pressure than a narrower master cylinder. This is what differentiates a power brake master cylinder (larger bore) from a manual brake master cylinder."

 

I would also go with the 1". The stock pedal at the correct height only has about 6 1/2" of usable travel, a full 7 1/2" puts it in the carpet. I personally agree with the wilwood guy about the 1 1/8" being too much and the 7/8" may be too small and result in a low pedal. 15/16" is mentioned several times in many different installs that I have read about, but that isn't one of their options. 1/16" isn't enough to loose sleep over.

 

My 1" bore wilwood doesn't travel far before it gets really tight. I like that. Makes the brakes feel firm and effective. Which they are.

 

Good info, I have a stock manual disk/drum M/C and still think my pedal moves to much... I thought it still needed bleeding... The brakes work great, so maybe a upgrade to a 1" bore will firm up the feel of the pedal....

 

With a disc/drum setup, make sure your rear adjustment is correct. If it drags lightly and locks up when adjusted, you might try a different shoe compound or adjustable proportioning valve. It should drag lightly, not bind, but not free either.

 

Thank you guys for all the feedback. I went with the Wilwood 260-8555-P Master Cylinder with 1" bore. It sounds like it will work for the right feel and pressure. I think the 7/8" would have provided the pressure but had a long throw to be able to apply the brakes giving it a soft feel.

Your feedback is not taken lightly. Experience outways all the speculation you could have.

Thanks again!!!!

 

Personally I would listen to the manufacture. They designed it so they are the experts. My company has sold Wilwood products for many years and their tech service is second to none and have never given us any bad advice. Generally with all ford vehicles the pedal ratio dictates a smaller bore especially for a manual system. So their advice of using a 7/8 bore is spot on.