Choosing Master Cylinder bore diameter

[jckkys]

My new WH catalog lists both 1" and 1 1/8" master cylinders. The choice is a small 1" for more fluid pressure or the 1 1/8" for more volume. The OE master cylinders were 1". To me the 1" would apply the brakes with more pressure and unless the drum brakes were way out of adjustment, volume wouldn't be an issue. But then why offer 1 1/8" bore master cylinders identified as heavy duty?

 

[nvrstuk]

SOooooo many variables here...

Do you have stock drums frt/rear?

Updated discs? frt only or all 4?

Hydroboost, manual or vacuum assist?

You mentioned "To me the 1" would apply the brakes with more pressure and unless the drum brakes were way out of adjustment, volume wouldn't be an issue"....

does this mean you have drums?

Are you ever going to upgrade your braking system?

tell us more...

 

[DirtDonk]

Heavy Duty is a moniker, and certainly indicative of it's being physically larger and having more flow than a stock one and with added capacity in the reservoirs. We've been using that name ever since we started carrying the larger master cylinder many years ago, to differentiate it from the regular ones.
But the name itself does not have anything to do with it being a more appropriate choice for every application.

We offer these larger-than-stock master cylinders primarily for use with boosters and for use with larger brakes, such as the GM disc front brakes. Especially and importantly when used with the larger GM-based rear disc brakes, in order to keep a firm short throw pedal action.
They'll work in a manual situation and you can still stop the Bronco, but it takes a noticeably harder leg effort to get the job done if used without a booster.

Yes, a smaller piston gives more output pressure than a larger one does, and that can be a good thing. Sometimes though you just need the extra flow from a larger piston to do the work properly. Different setups call for different options, as nvrstuk said.

Heavy duty in the name or not, If you're running manual brakes and/or drum brakes, I would not go over 1" diameter. As said it will still work, but it's not optimal unless you have strong legs and don't mind that hard pedal.

Paul

 

[jckkys]

The brakes are stock '77 disc/drum. The OE master cylinder has a 1" bore so that should be adequate volume, but I never tried any larger bore cylinders. I am using an after market master/booster bracket. This is necessary to clear the '73-'75 inner wheel tub, that itself is needed to use F250 long travel shock towers. The master cylinder needs to be able to fit between the wheel tub and the stock steel hood and look stock. This is basically a restoration with hidden modifications. In other words it needs to look original '77, but work better.

 

[DirtDonk]

I would stick with the 1" then. Not only are EB-sized boosters not the most powerful, but the larger piston is not needed in your case to achieve a firmer pedal (not usually anyway) because the stock size works pretty well with the Ford-sized brakes.
That's what I have in fact, though without the booster, and it works great too.

I'm even contemplating downsizing my '68's master to a 1" to even though I'm running GM front discs on that one. Pretty sure Doug used a full-size truck master so that it has the stock look but a larger piston.
It's a great firm pedal but I'd like to see just how much difference that 1/8" makes in power because I'm thinking I have some leeway in pedal travel.

And the WH HD master does not look anything like a stock one. Because that Bendix design was so popular on GM vehicles of all descriptions, if anything it would look like a stock Chevy!
Not what you're looking for I'm thinkin'

Paul

 

[jckkys]

Agreed. The stock brakes are very effective as OE. In fact the only problems I had over 43 yrs. were the result of negligent maintenance. It may be the 31" tires but the OE power brakes and steering worked very well for me. At least, that is, when well maintained.

 

[WheelHorse]

I used a 1" bore on my master as a replacement...works great with a little long of a throw, but very easy to apply brakes. Reminds me of classic GM brakes from the late 70's. You can lock 'em up with little effort.

 

[jckkys]

The stock '76-'77 brakes work as well as any. I prefer them to ABS. My '74 with non power drums was a little scary, even when new. By comparison the new '77 was a huge improvement. In fact I don't see any reason for a hydroboost, but then i don't see a reason for 33" and larger tires. With 31s the stock brakes are perfectly adequate.

 

[Broncobowsher]

Running the big bore T-bird calipers the 1-1/8" might make a better match.

But there are so many variables. If you thought you listed them all, you still missed a bunch. Even some very small stuff makes a big difference. And stuff that you never thought would be a variable in the braking system, it is a variable.

In general I see people put too large of a bore master cylinder on. The pedal stroke goes to none. The pedal doesn't feel like it moves, just pressure you put on it. And a lot of pressure that is. Some of the best braking improvments I ever did was to undo the big bore recommendations that were given to me. The smaller the bore, the better the brakes got.