Oversize master cylinder pistons?

Hello,

I have a half dozen original Wagner Lockheed master cylinders here, all opened up on the bench from searching for a good core to rebuild.  My two best candidates still have some pitting in the bores.  A heavy honing may get rid of most of the pits, but may leave the bore to piston clearance too large.  It is my understanding that the piston to bore clearance should be no greater than 0.005".  Does anyone know of an over sized piston kit on the market that would fit these master cylinders?   A piston kit that is a mere 0.010" or 0.020" oversize, would save a lot of these old cylinders from being recycled by allowing them to be bored oversize.

I was unable to find much on the web about it, it seems that most places are having bores sleeved on hard to find master cylinders instead, but the cost is at least twice that of a new replacement master cylinder.  If there are companies that rebuild or used to rebuild these master cylinders, I wonder if they have access to over sized kits?  I could only imagine that they would get a lot of rusted or worn cores, and would want to save as many as possible.

If oversize piston kits are not available, I have some connections to talented folks who can make over sized pistons, but I'd have to figure something out for the matching piston seals and rubber cups.

Any input would be appreciated.
Daniel


Edited by uglyjeep - 20 Jan. 2011 at 5:01am

Thanks Joe,

If there are no SAE kits available as over size specifically for these master cylinders, I am going to follow up on this metric lead and see where it takes us.  I imagine that there would be enough demand between here and the G503 to justify the effort of finding or making the parts needed.  A 26mm bore may allow me to save half or more of the cores I have.

Daniel

Jim,

I wouldn't think so... planning on turning pistons to match.

Thanks Jim for the lead on those. 

Joe, that is quite alright.  See if you can work with those, knowing your talents, I have confidence that this will work.  I did not see an application listed for the 1-1/32" cups, but if they were made for a specific car that can help for narrowing down a steady supply of them.  I will research this when I get some quiet time tonight after everyone here at the house is sleeping!

I also agree that new pistons will be needed, and should not be less than 0.005" of the bore diameter. 

What are your thoughts on seal that fits around the rear of the piston?  Looking at the seals on the rebuild kit pistons, all of mine have a cupped profile as well.  I am hoping that these 1" rear seals can be stretched a little over a slightly larger diameter machined groove in the piston, if matching 1-1/32" rear seals can not be found.

This is looking promising, I figured that the rubber parts would be the biggest challenge.  Metal pistons, while time consuming to make individually, could be rather economical if we could set up a large run of them if there was enough interest.

Daniel

it would seem to me enlarging the bore of a master cylinder of an already under sized system is not a good idea. the following is copied from the link at the end of this post.using the formula that is stated below increasing bore size of m/c to 1.0625 (1 1/32) would result in about a 11 1/2 % reduction in line pressure to wheel cylinders.

Rick

 

Brake line pressure is a different thing than the force you apply to the pedal. Force acts in one direction and is addressed in pounds. Pressure acts in all directions against surrounding surfaces and is addressed in pounds per square inch or PSI. "Levers" (brake pedals) can be used to change the force. Inside the hydraulic system, the surface area of the piston is what is affected by pressure. Decreasing the bore size of the master cylinder increases the pressure it can build. Pistons in master cylinders are specified by bore size. But there's a hitch: The area of a circle (or bore) is Pi–R-Squared. The area of the piston surface increases or decreases as the square of the bore size or diameter. For example, the area of a common 1-1/8-inch master cylinder is approximately 0.994-inch. The area of an equally common 1.00-inch bore master cylinder is approximately 0.785-inch. Switching from the larger master cylinder to the smaller version will increase the line pressure approximately 26.5% assuming that pedal ratio hasn't changed.

http://www.automedia.com/Better_Brake_Performance/pht20080501gh/2

Edited by Harveynailbanger - 22 Jan. 2011 at 12:31am

Thank you Rick, you bring up an excellent point.  I had worked some numbers on this when I first thought of reboring the master cylinder, and then I redid them using the cups that Joe was thinking of using.  I came up with a different numbers for line pressure reduction.  So let's work through this and see if I am missing anything here.  If I miscalculated somewhere below, please let me know, I respect what you are trying to say, because any modifications to factory safety systems deserve a great deal of thought and research.

Grab some coffee or an oat soda, and let me know if this makes sense...

Pressure = Pound Force / Area

Areas of master pistons as follows per pi * r^2:
1.000" piston is 0.7854 in^2
1.0236" (26mm) piston is 0.82294 in^2
1.03125" piston is 0.83525 in^2

Lets pick a constant pound force of 100 acting on the master cylinder piston for each scenario (which, by a rough measurements of the jeep brake pedal arm, may be like applying just over 22 pounds of force with your foot to the brake pedal), and using the formula above for pressure at the output of the master cylinder, P=F/A, in pound force per square inch (psi):
1.000" piston.............127.324 psi
1.0236" piston...........121.515 psi
1.03125" piston.........119.725 psi

If these numbers are close, then a 1-1/32" bore would only be a 6% decrease in pressure, and our initially suggested 26mm bore would only be about 4.6% decrease.  Granted, any pressure decrease from the master cylinder is not ideal for the cj2a, but I'd be willing to argue that even a 6% decrease would be tolerable, and perhaps hardly noticeable, provided that the rest of the system is in good working order.  For example, if the stock master cylinder were honed to 1.005" during a standard reconditioning job, the resulting pressure would drop about 1% using my calculation methods above.  So by using over sized pistons as we proposed above, compared with running a reconditioned master with a 0.005" over size bore, only results in output pressure differences of 3.6% to 5%.

More importantly, our calculations above are based on a single value for pound force acting on the piston for the sake of comparison, when in fact this value obviously is not a constant.  If we end up with a 6% reduction in line pressure due to an over size piston, we simply need to press harder on the brake pedal to compensate.  How much harder?...you guessed it, 6%!  Now, I couldn't find any info yet on what the maximum brake system pressure on the CJ2A was supposed to be, so I did some crude reverse calculations.  There is a popular formula that states that:

(brake system Pressure * Area of master piston) / pedal effort = the pedal ratio

I know that the pedal ratio is close to 4.5:1 on the CJ2A, and a 1" master piston Area is 0.7854 inches square.  Now let's suppose, in a worst case scenario panic stop, you were able to muster 200 pounds force on the brake pedal (this is a total shot in the dark here, I am not sure most folks could even do this much at the pedal!?), that would result in a system pressure of about 1150 psi.  So maybe this is close to a realistic maximum system pressure number that Willys or Wagner Lockheed intended, but I'd like to investigate this further.  The point that I am trying to make with this hypothetical case, is that if I had the 1.03125 master cylinder, I would only have to apply another 12 additional pounds of force on the pedal to reach the same seemingly excessive system pressure.  This doesn't seem like much, especially since most of us get a decent leg workout every time we drive these old contraptions!

I am avoiding looking into wheel cylinder changes, simply because it is late at night, well actually early in the morning!  You are correct in saying that altering wheel cylinder size can help compensate for the over size master piston.  I would venture a guess, that by merely honing all four wheel cylinders a few thousandths, one can shift the pressure percentage ratios back a little bit.  I also have had a fellow tell me once that one can run 7/8" wheel cylinders in the rear of a cj2a out of a Willys truck or wagon, instead of using the 3/4" cj2a versions.  I have no verification if this modification is feasible, but it may be another idea to alter the braking performance of the 2As.

Please let me know if I missed something, (it has been a long week with little sleep!)  Again, I very much appreciate coments and criticisms. 

Daniel


Edited by uglyjeep - 22 Jan. 2011 at 7:50am

here are some addt'l calcs:

Edited by Harveynailbanger - 22 Jan. 2011 at 5:25pm

Sorry for the obscure reference Rick, oat soda is slang for a beer, got that from The Big Lebowski movie.

Other brake enhancements may be worth investigating, but I only want to focus on a way to recondition an otherwise worthless master cylinder with this post, and discuss its effects when used with the stock brake system components.  The more modifications made to the system, means trade offs elsewhere in the system, just as you mentioned.  This is something I am trying to avoid, as I have already "chased that rabbit" while designing custom brake systems for the rockcrawlers that I used to build, it is time consuming, intellect taxing work...and perhaps unnecessary in this case.

Certainly, everyone needs to make their own competent, informed decisions when it comes to any brake system changes.  That being said, one of the reasons that I feel that the slight master cylinder change that we are investigating does not seem to warrant any further modifications has to do with simple human variation.  Say, for example, that you asked 100 people to sit in the same jeep, and press the brake pedal as hard as they can, and that you had a pressure gauge in the brake system to measure the pressure that each person created in the brake system by doing so.  I would be willing to bet that the pressure values would all vary, and it would seem likely to me that they could vary by way more than 6%.

Personally, I think that about 0.010" oversize would be ideal, but we may be limited by what is economically feasible in regards to seals and cups.

Daniel