USING GMC WHEEL CYLINDERS ???

[MIDNIGHT RIDER]

It seems this was discussed some years ago; however, I can't find it.

It is a common up-grade on the Dodge sites to replace the original 1" (bore) or 1-1/16" wheel-cylinders with the 1-3/16" GMC wheel-cylinders.

Whereas, the Ford and Dodge cylinders are LEFT and RIGHT specific, with the brake-lines entering at a corresponding angle; the brake-lines enter the GMC cylinders straight in, perpendicular to the bore; thus, the GMC cylinders can be switched side-to-side.

The brake-lines are easily re-shaped to accommodate the GMC orientation.

It is also my understanding that the GMC mounting-bolt pattern is the same as the Ford and Dodge.


My DRW F350 has factory-issue 1-1/8" bore cylinders, already larger than the 1-1/16" Dodge.

Will I gain or lose by switching to the 1-3/16" GMC cylinders ??


Has anyone already actually done this on a Ford of comparable vintage to mine.





I do know one thing; a nearly worn-out Chevy/GMC of late-70s vintage will stand on it's nose; whereas, most well-maintained Fords I have driven want to run through the bridle.


Thanks for reading.

 

[79jasper]

I don't have the math to back anything up.

But since the master cylinder can only put out x amount of fluid, by going to a bigger volume wheel cylinder, wouldn't you require more pedal travel and actually lose braking force?

If one swapped to the SD(?) Master cylinder which IIRC puts out more volume(?) I could see a difference if you did the wheel cylinders.

I dunno though. Just my thoughts on it.

 

[Wyreth]

I don't have the math to back anything up.

But since the master cylinder can only put out x amount of fluid, by going to a bigger volume wheel cylinder, wouldn't you require more pedal travel and actually lose braking force?

If one swapped to the SD(?) Master cylinder which IIRC puts out more volume(?) I could see a difference if you did the wheel cylinders.

I dunno though. Just my thoughts on it.

It's late, I'm not going to do the math, but you're only looking at a few thousandths more pedal travel. For a very minor increase in force multiplication. My opinion is that it's simply not worth it for 1/16" larger diameter cylinders. Maybe if you had to change them anyway, but I doubt you'll notice a difference. Especially since the rear brakes actually don't do a whole lot of the work stopping you.

The SD master is a larger diameter cylinder. So while yes it puts out more volume, it actually decreases the force multiplication at the wheels. Giving you less braking pressure.

The simplest and cheapest upgrade for the brakes on these trucks is to hydroboost.

 

[icanfixall]

Interesting information here. My thoughts are whay not try it but I sure would upgrade to the hydroboost and the larger 1 5/16 master cylinder. Not the idea that the larger bore ms will require more peddle force is not exactly right. Please remember the peddle has a different pin location for the mc pushrod. Its actually closer to the fulcrum so the advantage of peddle force has increased but the travel and increased too. But that seems to have canceled out too. More fluid volume being pushed and more pressure pushing it. I just know the hydroboost with the larger mc really stands these rigs on their nose when stopping compared to the vacuum brakes we had factory. Those damn things only worked half way right. Most of these trucks don't have the rear brakes doing any stopping because of the fake self adjusters never working correctly. That has been road to death on this forum too..

 

[Jake_IN]

Well just for some quick math here lets just calculate the difference in force you would see from each cylinder with the same pressure.

We know that F=P*A where F=force P=pressure and A=area

So for the Ford cylinder we have.

Given: 1.125 inch dia. cylinder and an assumed pressure of 1200PSI (just a wild guess for this math)
Find: Force

Solution: (1200lbs/in^2)*(pi*(1.125/2)^2)in^2 = 1192.82 lbs

Now for the GM cylinder.

Given: Cylinder dia. of 1.1875 inches.
Find: Force

Solution: (1200lbs/in^2)*(pi(1.1875/2)^2)in^2= 1329.04lbs


Here is a graph that shows the difference in pressure and force. X-axis is brake pressure (Psi). Y-axis is force (lbs) between the two cylinders. I really hate this graph because it has no labels but i think it gives you an idea of the difference between the two.

You must be registered for see images attach

And yes the one thing I do miss about my old K20 was its ability to plant your face into the steering wheel with minimal force on the brake pedal.

 

[typ4]

Ok , heres the seat of the pants reply.
I put 1-1/8 cylinders on my rears so they WOULD do some of the work, the utility box weight mandated it. It works like night and day, I used to go through pads every 7-8 months, now 2 years since the wc mod. I have hydro with the stock master filed to fit the studs, with new adjuster parts that actually work, I have 2-3 inches of pedal travel and it plants you.
And as far as the chevys having better brakes I dunno, I drove a 2wd dually for 6 years for a company fuel truck and those brakes sucked also till I did an upsize from 1 to the 1-3/16 to make the rears work and not slide the front on wet pavement. Steering on slick roads while braking is kinda important IMO.

 

[riotwarrior]

Hmmm...interesting thread!

 

[LCAM-01XA]

Will I gain or lose by switching to the 1-3/16" GMC cylinders ??

Has anyone already actually done this on a Ford of comparable vintage to mine.

There is actually no need to use GM wheel cylinders, Ford used 1-3/16" from the factory or dually trucks with the Sterling rear axle. However one needs to be aware that there are two kinds of those, bricknose ('87-'91) trucks have the cylinders machined and threaded for 3/8"-24 line nuts (the standard 3/16" line flare nut), whereas some later models still use 3/16" brake line but this time the nuts are of the larger 7/16" fine tread variety. Why the change was made idk, I'm only mentioning it as a PSA. In case of Midight Rider's truck, factory wheel cylinders for an '87-'91 dually F350 should bolt right up and not even require reshaping of the brake lines. RockAuto often has cylinders made by Bendix or Allied Signal right here in the USA, sometimes on sale for some ridiculously low price too.

 

[MIDNIGHT RIDER]

My original logic agreed with 79jasper's very logical reasoning.

However, although I never saw any math or science to explain it, the Dodge guys who had made the switch seemed to be well pleased with the results.

Thanks, JAKE_IN, for doing the math; if I understand the mathematical results correctly, then the slightly larger bore cylinder should be the better performer, right ??

One other point: (and it could very well already have been figured into the equation) there are TWO "pistons" in a wheel-cylinder; so, would not the difference be doubled ??


Thanks, LCAM, for enlightening me as to the existence of the FORD 1-3/16 cylinders.

Thanks, everyone, for all your input and typ4 for your real-world experiences.





Not to de-rail my own thread, but, the Dodge guys with 2-1/2" brakes are also truing the drums and using 3" shoes with good results; of course that does not apply to those of us already with 3" shoes.

 

[Jake_IN]

Thanks, JAKE_IN, for doing the math; if I understand the mathematical results correctly, then the slightly larger bore cylinder should be the better performer, right ??

One other point: (and it could very well already have been figured into the equation) there are TWO "pistons" in a wheel-cylinder; so, would not the difference be doubled ??

No problem sir, i actually kind of had fun doing that math this morning. At least I enjoyed it a lot more than the strength of materials homework I've been in the lab working on for most of the day. Yeah basically the graph confirms what we would expect (because of the larger surface area). However all that math was done with me completely forgetting that there are two pistons in there (was working on my first cup of coffee at the time). Here is another graph factoring in the second piston. Basically i took the above formula and threw a 2 right before pi. I think this is correct. However I would welcome someone with better knowledge of hydraulics to check my work.

You must be registered for see images attach

 

[MIDNIGHT RIDER]

I readily admit to being WAY the dumbest one in this discussion; but, so far as "seat-of-the-pants" effect, would not the increased area of the two pistons pushing both shoes against the drum double the mathematical gain ??

 

[Knuckledragger]

Look at the graph. It tells all you need to know. Yes, there is a difference, but it is so small that unless your brake system is on the very edge of not stopping to begin with, your foot and leg will not likely recognize any change. Try putting one in one side of the truck and see if you start skidding on that side all the time.

 

[LCAM-01XA]

I readily admit to being WAY the dumbest one in this discussion; but, so far as "seat-of-the-pants" effect, would not the increased area of the two pistons pushing both shoes against the drum double the mathematical gain ??

Yes, yes it would - but only if you're comparing two pistons and two shoes vs. one piston and one shoe (or one piston and two shoes cause one piston is rust-frozen in place and don't move and the other piston is doing double duty). If comparing a pair of 1-3/16" pistons to a pair of 1-1/8"pistons, there will be a difference but idk how noticeable it will be, likely driver won't be able to tell the difference unless truck is loaded very heavy and every little bit of extra braking power helps.

 

[Jake_IN]

Yeah LCAM I think you are right that due to the pistons being independent from each other the original math will still hold true. The original math is just the force EACH brake shoe is seeing. However knuckledragger is right in that the difference is minimal. With a brake line pressure of 2000PSI you only see a 200 lbf difference between the two types of cylinders.

 

[icanfixall]

No vacuum brake setup can make 2000 lbs of hydraulic pressure in the braking system. Only yht hydroboost brake system has that potential pressure. And even then its not used for every stop but its there if needed. Thats the part I like. Knowing you have the extra stopping power is reassuring to say the least.. We still need to remember that braking surface has not changed so we can't safely stop the world from turning with our brake surfaces. But maybe an upgrade to discs all the way around will...