Upsizing my MC - how much?

I was sold too small of an MC for my disc brake swap on my 67 Dodge Coronet. It's 1 1/32, and with 2.75" piston Wilwood GM Metric calipers in the front and 15/16" wheel cylinders in the rear, my pedal is long and mushy and I have no ability to lock the front brakes (manual). I had a harder pedal and more power when I was on all-drum.

If I clamp the rear flex line, a good 80% of my pedal travel is just getting the front brakes from rest to loaded against the discs (this is not a bleeding issue, I can't pump up from this). I do have stopping power, but effort vs output really ramps up beyond moderate braking. Nothing happens with the pedal until it's almost parallel with the top of the gas pedal (I have moved the pedal by hand and listened in my garage, that is just the stroke required to take up the front brakes).

I have been trying to figure out what the best size is to step up to. I have a universal/GM-style two-hole master so I can pick from a lot of stuff. How do you determine this? I can use 1 1/8 from a 70s D-series, or 1 1/4 for an 80s K1500 with regular OE Metric calipers and huge rear drums, etc. I don't think 1 1/16 would be a large enough step up considering how long and squashy my pedal is now.

There are a few people on here who know all the math of working this out but I'm not one of them I think you'll need to know the sizes of the brake pistons (EDIT: As in, both the ones on the car now and the ones your MC was intended for).

I'm pretty sure 1/16" is indeed too small of a step-up for the magnitude of your problem though, I know what that difference feels like. The 1 1/4" MC seems like a decent bet.

Diameter * Diameter * 3.14 / 4 gives you the face area of the piston in the bore.

Double the area and you double the volume of fluid moved by the same stroke. So this means you'll move the pedal half as far, and it will take twice as much foot force to lock the wheels.

Apply that ratio concept to the various bore sizes you are considering.

Very quick and casual, you're talking about a 20% increase in fluid flow with the 1-1/8 cylinder, and a near 50% increase with the 1-1/4 cylinder.

From your description, I've personally be inclined to go to the 1-1/4 bore myself.

Look up dimensions for MC that came OE With calipers of that size, and get one of those.

Keep in mind that a typical boost ratio for a passenger car is about 6:1, so going manual means you'll need to apply 6x more leg muscle to get the same decel. All that additional pedal force will be reacted through the firewall, so you may experience additional flex that will cost you some pedal travel. I learned this firsthand when developing the manual brakes on my v8 944, and had to provide significant bracing to reduce firewall flex.

IMO those calipers would be adequately supplied by a 27 mm MC (approx 1-1/16") if everything else is right. Maybe 28.6 mm (1-1/8"). Look up mid-'90s Chevy s10, which would have had similar front calipers with rear drums.

You will need an adjustable prop valve in the rear circuit, and you will need to seriously reduce the pressure to the rear drums, as (here's what a lot of people don't grok) the torque output per unit pressure input is significantly lower for a disc than a drum of similar diameter. As a result, replacing front drums with discs shifts the brake system bias significantly to the rear.

June 2008 GRM has all the equations. Is that article in the online archive?

Ratio between the old and new master cylinder areas = force and travel increase or reduction.

First check - do you have the front and rear lines hooked up the the correct outlets? It does matter in a lot of cases, even if you don't have a stepped master.

In reply to Keith Tanner:

Yep. Not sure why I didn't mention that first. Keith wins!

Yes the lines are hooked up right. I have an MBM prop valve on the rear circuit. This isn't a brand-new installation, I'm just driving the car more. I can drive the car like a normal person and brake pressure is reasonable - once you get into "stop fast" territory I start having to use more leg than I'm comfortable with. I'd rather not have to stand on the pedal to stop quickly.

After a lot of careful twinkle-toe observation I think I'll give the 1 1/4" MC a try. If I ever install a booster I might have to downsize, but I would like to be able to get better one-foot effort so I might as well go big.

Just understand it's the square of the ratio, so that's a pretty big change.

Larger MC bore requires MORE pedal force required to reach the same decel, which sounds like the opposite of what you want.

I found some interesting staggered MCs when looking through various years of GM stuff with Metric calipers. I found a manual one (Dorman M39589) for an 82 C10 pickup (with 15/16 or 31/32" wheel cylinders) in with a 24.4 mm front/36 mm rear. That equates to about 31/32in front and a big old 1 13/32" rear (3/8 for an inaccurate round with more familiarity).

I assume these are all the "quick take-up" masters meant to be used with low-drag calipers. I learned these use an internal valve to give some of the huge rear piston's volume to help fill the front to make up for the extra retraction built into them - but then there's a very small front cylinder for the majority of the actual working brake travel.

While my pedal travel is annoying, I am ultimately running out of leg power for resultant braking power which means I need more line pressure, right? Big MC = less pedal-to-pad ratio, but lower PSI. If I have to stand on it now it'd be worse with a larger MC.

If I go back to late 70s trucks, manual MCs are 1" with construction that looks like this: http://www.rockauto.com/en/moreinfo.php?pk=967088&cc=1030329&jsn=958 I assume the big stonky rear reservoir is meant for rear wheel cylinder volume. Maybe this would be a better bet?

If these Wilwoods are over-retracting that's a different matter (and maybe this MC being busted for all I know), the most important thing is more braking powah.

AngryCorvair wrote: Larger MC bore requires MORE pedal force required to reach the same decel, which sounds like the opposite of what you want.

Verily I have learnt the oppositeness of my ways

Is adding power brakes a possibility? That would let you run a bigger master or different pedal ratio and cut down on the pedal travel without having the effort go sky-high.

Sounds like your system is missing a residual pressure valve in the front. to keep the calipers from retracting so far.

Your 1-1/32in MC displaces 0.84sq in of fluid.

1-1/16 would be 0.89sq in(a 6% increase).

1-1/8 is 0.99sq in(18% increase)

1-3/16 is 1.11sq in(32% increase)

1-1/4 is 1.23sq in(46% increase)

A small change in MC diameter goes a long way. I'd start with the 1-3/16 based on what you're saying.

On my Jeep I went from a stock 1in dia MC when I swapped 1 ton axles, to 1-1/4 MC... a 56% increase in displacement. It went from pedal too long and soft(much like yours) due to the larger caliper pistons with stock master, to very very firm. Yes it "feels" good but you cannot stand on the brakes and have them lock the wheels up like you really should be able to. I needed either a smaller master or a larger booster to power it. I personally think the 1-1/8 would have been the way to go in my case(which would have been a 25% increase in displacement in my application).

IMO I'd go with the 1-3/16.

Pedal ratio is something else to consider, although as I thought about it, would changing the pedal ratio be at different than changing bore size??

jfryjfry wrote: Pedal ratio is something else to consider, although as I thought about it, would changing the pedal ratio be at different than changing bore size??

Functionally they do the same thing, changing pedal ratio is generally much smaller adjustments.

Also it won't help if you bottom out the master cylinder first

jfryjfry wrote: Pedal ratio is something else to consider, although as I thought about it, would changing the pedal ratio be at different than changing bore size??

They both change the leverage, but if you do it by changing the pedal ratio then you run the risk of running out of travel in the hydraulic cylinder before you've got the pressure you want.

The proper MC will have significantly higher effort -- why not put in a booster too? IMHO big heavy sedans with disc brakes need boosters.

The Dorman M39589 will certainly provide a lot more pressure to the front brakes than the rear, but with only a 15/16" bore for the front, it will probably have pretty long pedal travel.

Thanks for all of your input guys - good manual brakes is not sounding possible for my configuration. If I get the line pressure I need, I'll probably risk running out of travel in extreme situations. I'll have to step up and install the booster I've had laying around for awhile. I'm going to see how it works with the current MC before I pick which to switch to - the stock Mopar power MC in the 70s was around my size, I believe, but I've never driven a 70s power brake Mopar so I have no idea what they feel like.

Many pedals are drilled with power and manual holes so you may want to check your master push rod is in the right hole

It is, it is a factory manual drum car.

Secret_Chimp wrote:

AngryCorvair wrote: Larger MC bore requires MORE pedal force required to reach the same decel, which sounds like the opposite of what you want.

Verily I have learnt the oppositeness of my ways

Smaller master cylinder requires more stroke for the same fluid movement. Beware this. The excessive length of the pedal travel is what I was focusing on.

You can always push harder or add a vacuum booster. But you really can't band-aid it when you stroke out the master cylinder and still haven't engaged the brakes.

I think you need to look at the pedal ratio option first. Bigger MC moves more fluid per stroke but from the art led I've read can reduce force applied at the piston. It sounds like OP is getting pressure, especially when the tears are reduced in fliw, but the pedal travel is worrisome. Altering the relationship between the pedal pivot and the MC may get a faster stroke .