Help choosing size of MC

[jamieg285]

I'm upgrading the front brakes on my currently stock 2nd Gen F-body. Retaining stock rear drums and booster. Usage will be spirited street driving.



I have had conflicting advice over whether to go for a 15/16" or 1" MC. From the research I have done, either should be fine, but will have different pedal travel/hardness.

What I want to know is, if I chose either one of them are there any options to modify the feel if I am not happy with it, other than swapping to the other sized MC. i.e. change pedal ratio, proportioning valves.

If there are such options, which would be the better size to start with?

Thanks,

Jamie

[Apogee]

First, you mention that you're sticking with the stock rear drums, but don't share what are you doing in terms of "upgrading" the front disc brakes. Are you sticking with the stock discs and adding new calipers, pads or rotors or swapping them out for something else entirely? Clearly your MC bore size requirements will depends on the slaves, which at this point we don't know. Beyond that, you'll want to take your engine's vacuum level, booster size/configuration and pad compound into consideration as well as your personal preferences with respect to pedal feel. Without a more complete picture of what you're trying to accomplish, discussing a 1/16" (~11%) variation in MC bore size seems a bit like putting the cart ahead of the horse.

That said, one of the easiest things to change on any given brake system is the MC bore size, which will impact pedal travel and effort inversely to one another. A smaller bore will require less effort and more travel to actuate the brakes and vice versa. A proportioning valve won't impact pedal feel one way or the other and pedal ratio tends to be more difficult to change without potentially creating angularity issues with the booster.

Tobin
KORE3

[jamieg285]

I've got a Baer Track Plus kit to fit, which I think is 13 disc and an C4 style caliper. It's only for street use, so won't have race pads. Booster is 11" stock. Not sure what the vacuum level is (car in bits), but it's not a particularly wild engine, so should be reasonably high.

From the comments so far, I'm leaning towards the firmer pedal with a 1" MC.

[Mach1_Ron]

1" sounds good with Power Brakes. Why don't you ask Baer an see what their thoughts are also? when I was trying to get my brakes sorted out, just a few months ago, I talked to Wilwood since they are who made my brakes and they were extremely helpful in getting everything working together.

[RobNoLimit]

While I agree with Tobin about the importance of knowing all of the parts used, and the required fluid volume needed to move the pistons, here is some generic numbers for you to consider.

Lets say that the OE pedal ratio is appx. 6-1 (that's close), and I like to use 60 lbs. as a standard for "foot pressure input" on the pedal. The 1" OE booster is about a 1.5 - 1 multiplier. So, 60 lbs. x 6 x 1.5 = 540 lbs of input pressure into the master. A 1" bore master has .785 sq.in. of piston surface. And a 15/16" master has .689 sq.in. So, a 1" bore master with 540 lbs input will build 688psi. line pressure. A 15/16" master with the same input will build 783 psi.

If you had a 1" and wanted more pressure, changing the pedal ratio to 6.5-1 would build you 745 psi.

If you had a 15/16" master and wanted less travel, a 5.5-1pedal ratio would net 718 psi.

There are variables here with the Power assist. Higher vacuum = higher boost percentage, and lower = lower. Notice that the full range that I laid out is within 14%, yet these will all feel really different. With the booster, I would start out larger, 1" or 1 1/16". Ask the guys a Baer, I'm sure they have worked on the same combo.

[jamieg285]

1" sounds good with Power Brakes. Why don't you ask Baer an see what their thoughts are also? when I was trying to get my brakes sorted out, just a few months ago, I talked to Wilwood since they are who made my brakes and they were extremely helpful in getting everything working together.

Baer are the ones that suggested the 15/16", but that was with less information than I've given here. Whilst researching I've seen a few posts where that has proved to small and they had to upgrade to a 1"

With Tobin's comments suggesting there isn't really any adjustability, I think I may be better going with the 1", which more people seem to recommend.

[jamieg285]

While I agree with Tobin about the importance of knowing all of the parts used, and the required fluid volume needed to move the pistons, here is some generic numbers for you to consider.

Lets say that the OE pedal ratio is appx. 6-1 (that's close), and I like to use 60 lbs. as a standard for "foot pressure input" on the pedal. The 1" OE booster is about a 1.5 - 1 multiplier. So, 60 lbs. x 6 x 1.5 = 540 lbs of input pressure into the master. A 1" bore master has .785 sq.in. of piston surface. And a 15/16" master has .689 sq.in. So, a 1" bore master with 540 lbs input will build 688psi. line pressure. A 15/16" master with the same input will build 783 psi.

If you had a 1" and wanted more pressure, changing the pedal ratio to 6.5-1 would build you 745 psi.

If you had a 15/16" master and wanted less travel, a 5.5-1pedal ratio would net 718 psi.

There are variables here with the Power assist. Higher vacuum = higher boost percentage, and lower = lower. Notice that the full range that I laid out is within 14%, yet these will all feel really different. With the booster, I would start out larger, 1" or 1 1/16". Ask the guys a Baer, I'm sure they have worked on the same combo.

Interesting, thanks for taking the time to work these things out. I think this goes to re-inforce that 1" would be the better choice.

Going on your comment about vacuum level = boost level, I assume that translates to the lower the vacuum, the harder the pedal?

[RobNoLimit]

Yes. Lets say the vacuum is low, and the booster amplification is 1.3-1

Then 60 x 6 x 1.3 = 468 lbs input, with 1" bore is 596 psi. So, if you needed say 750 psi for a hard stop, that would now require 75.4 lbs of foot pressure.

[Apogee]

Given that you are running what are PBR C4 front calipers with relatively small piston areas, twin-38mm pistons net an effective piston area of about 3.50 square inches, I would actually recommend the 15/16" bore unit over a 1" in this case because the calipers will require about 12.5% more pressure to achieve similar brake torques as a C5/C6 setup. FWIW, the 1992-1996 C4 Corvettes also used a 15/16" bore MC paired with a single-10" diaphragm booster IIRC, although I don't know what the pedal ratio on those applications were off hand.

Tobin
KORE3

[jamieg285]

Thanks for all the help so far - I think I have now decided on an MC - 15/16 from CPP.

Next question - this is a disc/disc MC. As I am retaining stock rear drums, do I need to get a manual prop valve for it, or can I plumb it into the existing stock one?

[jamieg285]

Actually, let me re-phrase that question:

Will I *need* a manual prop valve, and even if I don't and can use the original stock one, would there be any benefit to swapping them and having some tuning ability?

[Apogee]

While an adjustable proportioning valve would typically be preferred in an application like yours, your factory fixed disc/drum combination valve may work just fine for you, so I would suggest you test it and find out.

What you're lacking with a disc/disc master cylinder is a 10 psi residual pressure valve (RPV) for the rear drum brakes. This RPV serves to maintain a small amount of pressure on the rear wheel cylinders which keeps the cup seals "inflated" when you release the brakes, and the drum return springs collapse the wheel cylinder pistons. Without an residual pressure, some seal designs are prone to folding which can allow fluid to leak out of, and air to leak into the system, resulting in a mushy pedal and ineffective rear brakes.

You can purchase 10 PSI RPV's for about $20 from a variety of sources, including Wilwood.

Tobin
KORE3