Technical Master Cylinder Issue

There really is a difference in stroke, or fluid volume, on many if not most dual OE master cylinders, and is why many on here who plumb the fronts to the wrong master port have brake issues. The reason is the result of certain DOT and OE tests that center around fluid volume requirements matching brake displacements.
I've said it before, and I'll say it again; know how the master cylinder you are using is designed to be plumbed.

 

I’ve never seen one that there is a difference, please explain how there is a difference in stroke on a single bore master? Hydraulics don’t work that way...there would have to be a difference in primary seal size to get a different stroke

No disrespect meant Bob, I’m willing to be wrong


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I don't use proportioning valves, I like distribution blocks. Not all PVs may work with any master.

Your best bet is to get one matched to the master if you are set on using one. I like to know what my master came from, I'll buy them at a parts store. The one on my car is for a 1972 Mustang. So I can get one fast anywhere.

 

Piston length/design, comp port location and piston spring tensions are three items that come to mind, maybe more. I didn't design master cylinders; I only worked with and for the guys that did. An example would be a master cylinder total bore length of 1.5", with the primary piston capable of moving 7/8" and the secondary 5/8". This ratio was used during all normal braking but with less actual piston movement. The ratio provided the necessary fluid volume for the dissimilar front and rear brake fluid requirements. Are we clear!

 

Great information @V8 Bob and @David Gersic

 

I get compensation port location although again I e never seen them varied. Otherwise once the primary seals cover the ports they are both moving the same distance. I’m sorry but the springs are no where near stuff enough to change application, they are to help with piston return.

I would like to see literature referring to this, until then I stand by what I said. Yea I’m stubborn.


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So, if you crack the line at the master cylinder at the end of the travel for the front brakes, will the pedal
drop more?

 

Just to add to what Gordon C mentioned.
I also had to make a longer pin for my upgrade than what came in my kit.


And just for the sake of visuals I am adding a shot of what I have.

 

Fargo,,,thats a good one
Lol,,,,I got tickled there .

Every time before after a few posts you use the F word and then storm off .

Tommy

 

I don’t understand? I guess I shouldn’t care but the misinformation is frustrating.

I have a lot of respect for v8Bob so I’m asking him for more information, I’m sorry I don’t understand how two pistons the same surface area can move different distances, doesn’t work.

I don’t ever “storm off” I simply give up trying to help. I said a while ago I’m not sure why I expend so much energy and time trying to help strangers who usually aren’t interested. I’ve said it before all of my junk stops fine.


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It does make a difference which port goes to the front and back brakes.

 

Both resevoirs are the same size

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Normally I buy ones like this as well.. 1st time I have honestly reused one but it was working when I took it off but didnt know what it came from..or think to pay attention to which side was plumbed where because they were equal size reservoirs. I will be getting ahold of a new MC just to have it this weekend when I really break into this hopefully aremed with a good anount of troubleshooting ideas

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I thought of that and the pedal functions,the same as if i were bleeding it at the calipers

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Ohh thats actually really helpful...i need to look at where my fittings are in relation to the ports in the MC...it might be of mention that my MC did have one of the blocks on it but I removed it and went with straight prop valves...they fit better in my application

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My proportioning valves are on the individual circuits I dont have that block attached to my MC..hard to see in the pic but if you zoom in you will see the 2psi wilwood PV going towards the front of the vehicle..have a 10psi going to the back

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I might be guilty of thinking about the OPs problem in the simplest terms but here is what I think. I'm assuming he changed something in the system and has air in all 4 corners. As he bleeds the back, the pedal goes to the floor because the fronts haven't been bled yet, which allows the front circuit to act as if it was open. As he bleeds the back and now has the rear brakes functioning, shouldn't the master/pedal only go half way...or offer resistance? The back brakes are now functioning and air free. So he is gaining pedal as he eliminates air from the system....seems normal to me. What am I missing?

 

I do want to thank all of you for chiming in..I am actually learning more about MCs than I previously knew..All very helpful! Please let me know if any specific pics would help any of you..i will post them as soon as I can.

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Then I was going to have you try cracking the one on the rear....if you are having travel limited by the function of the master cylinder this will tell you that or not.
It's called troubleshooting and process of elimination thru gathering of data
It might be any of the components in the system. You have proportioning valves between the slave cylinder and the master cylinder too so maybe, maybe not. But until you try...

 

All of this is a new,install the truck has not been on the road since I tore it apart...i have gone all around the truck and bled everything...i get no more air out of the system amd any bleeder I open I no longer get air...my belief is the system is free of air but still seems soft to me...and my concern is I am not getting a long enough throw from the front to actually bleeed the air fromt the front..



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I agree I didnt crack the fitting for the rear circuit as the brakes were functioning as expected in the back but at this point I will certainly try it..

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You do know that it doesn’t use the full stroke when you step on the pedal correct? That picture proves nothing, the only thing that could change how much the secondary piston moves is either the size of it or where the vent port is in relation to the primary seal.

Also he doesn’t have inline proportioning valves he has residue valves.


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The pedal doesnt truly go to the floor and stop...when doing the back brakes the pedal stops about an inch above the floor board...when,doing the front it stops about 4 in or so from the floor..

All brakes are bled as much as I think they can be...wheels all seem to stop but I havent pjt the weight of the truck behind them...i cant turn the wheels in front or back by hand when the brakes are applied I havent put any breaker bars on them to try and over come them by hand

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Then you just don't get it! Not going to argue, I'm done here.

 

Yes, I'm wondering about what is being called proportioning valves, if they are really residual valves...I would think no residual valve on the discs....only on the drums

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Ok...suit yourself

I still haven’t seen anything that proves me wrong however.

Good luck getting your problem fixed OP.


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Well thats a big screw up on my end sorry for the terminology mix up...yes they are residual check valves..the rule of thumb I have always gone by is that you have a 10# residual valve for drums regardless but if the MC is mounted lower than the calipers like under a floor then a 2# residual check valve is used...so in this case I have no proportioning valve in my system just residual check valves

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I have a manual, disc-drum, 1" bore master cyl. for a mid-70's F-100 on my '32. I also have a Wilwood proportioning valve on the rear drums. I have 11" drums and 11" discs. Typical hot rod stuff.

The master cyl. is mounted under the floor and backwards to the normal position, and I have the front discs plumbed to the large reservoir. Everything works fine. When I bleed the brakes, the pedal does go to the floor for either end.

After thinking about this thread, I don't have a shuttle valve between the front and rear reservoirs, like it would with a factory GM or Ford proportioning valve. So, if I were to have a line or wheel cyl. failure (front or rear), does that mean I would have no brakes?

I think so! How many of you have a similar set up?

 

I think you need residual pressure check valves on both ends (2# on the discs and 10# on the drums), if the master cyl. is below the wheel cyls., because it will suck air past the wheel cyl. seals if you don't maintain a slight pressure on the seal. If the master is above the wheel cyls., then gravity will keep a slight pressure on the seals and air won't sneak past.

Also, some master cyls. have a residual pressure valve in the ports where the brake lines screw into. I don't know which ones, but I would think a factory under floor unit would.

It sounds like those factory proportional valves do multiple things!

 

Yeah I have the residual on both ends just no proportioning valve...this looks,like a vette style MC but not certain exactly what it is...but I have a bunch of good ideas for this weekend to check out...will grab a portioning valve too..

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