Brake master cylinder - I don't know what I need to know...
Discussion
Evening ladies and gents. I'm after a bit of expert advice, or at least, someone to tell me if I'm missing anything incredibly important.
What I'm doing: I have a 1973 beetle chassis that's being converted into a... 'thing'. I need to change the standard tandem dual brake master cylinder for two individual master cylinders due to 'packaging issues'. It won't all fit as is.
What I don't know is, what I need to know to know if I'm doing it right.
My theory is:
The current tandem M/C is 19.05mm bore, twin 25mm throw. It has residual pressure valves fitted (obviously, because the car has drum brakes) and puts out equal pressure front and back, with different slave cylinders used to give the % of braking force split.
If I get 2 master cylinders @ 19.05mm bore and transfer the residual pressure valves in, and equally split the load from the pedal (bias bar set to zero) it should end up the same as having the tandem M/C right? And then I can further adjust bias bar to get a better split?
Am I missing anything important?
What I'm doing: I have a 1973 beetle chassis that's being converted into a... 'thing'. I need to change the standard tandem dual brake master cylinder for two individual master cylinders due to 'packaging issues'. It won't all fit as is.
What I don't know is, what I need to know to know if I'm doing it right.
My theory is:
The current tandem M/C is 19.05mm bore, twin 25mm throw. It has residual pressure valves fitted (obviously, because the car has drum brakes) and puts out equal pressure front and back, with different slave cylinders used to give the % of braking force split.
If I get 2 master cylinders @ 19.05mm bore and transfer the residual pressure valves in, and equally split the load from the pedal (bias bar set to zero) it should end up the same as having the tandem M/C right? And then I can further adjust bias bar to get a better split?
Am I missing anything important?
Are you sure that is a residual pressure valve and not a bias valve?
The tandem master valve is resilient in that if either brake circuit fails you can usually still get pressure on the other circuit. The balance bar setup you're describing wouldn't do that unless the balance bar runs out of travel - which it probably won't.
The tandem master valve is resilient in that if either brake circuit fails you can usually still get pressure on the other circuit. The balance bar setup you're describing wouldn't do that unless the balance bar runs out of travel - which it probably won't.
tapkaJohnD said:
make up a system that applies equal pressure to both masters. A rigid bar, not a balanced one
Surely a rigid bar would apply equal travel rather than equal pressure. (Not that I've ever seen one - I imagine that designing the pedal assembly to deal with the unbalanced forces might be tricky.)tapkaJohnD said:
Your bias bar is fighting a method of biassing the front/back split, different sized slaves, that it was not designed for.
Either:
make up a system that applies equal pressure to both masters. A rigid bar, not a balanced one
OR
use same size slaves front and back.
John
The bias bar will for the time being be locked once in the correct place to give as close to the original setup as i can hence mentioning the bias bar being set to zero. Another reason I'm doing it is to future proof it - it'll have disk brakes up front at some point.Either:
make up a system that applies equal pressure to both masters. A rigid bar, not a balanced one
OR
use same size slaves front and back.
John
Does the rest of the theory sound... sound?
Quote greenV8: Surely a rigid bar would apply equal travel rather than equal pressure. Both, surely? Why not equal pressure?
JOhn
Davi said:
The bias bar will for the time being be locked once in the correct place to give as close to the original setup as i can hence mentioning the bias bar being set to zero. Another reason I'm doing it is to future proof it - it'll have disk brakes up front at some point.
Does the rest of the theory sound... sound?
I don't understand what you mean by "balance bar set to zero". There is no 'zero'. And IMHO, not withstanding greenV8, if the balance bar is rigidly fixed, then the two masters will get equal pressure.Does the rest of the theory sound... sound?
JOhn
All sounds fine to me OP, as said on any non s
t pedal box with bias bar if you lose one circuit the relevant master cylinder will bottom out and you'll still be able to apply pressure to the remaining circuit. So long pedal but still some brakes, just like the tandem.
IIRC for it to be road legal you must remove any adjusters and lock the bias bar (so it can't be turned)
t pedal box with bias bar if you lose one circuit the relevant master cylinder will bottom out and you'll still be able to apply pressure to the remaining circuit. So long pedal but still some brakes, just like the tandem. IIRC for it to be road legal you must remove any adjusters and lock the bias bar (so it can't be turned)
tapkaJohnD said:
I don't understand what you mean by "balance bar set to zero". There is no 'zero'. And IMHO, not withstanding greenV8, if the balance bar is rigidly fixed, then the two masters will get equal pressure.
JOhn
If the balance bar is set central (presumably what the OP means by zero) then the pedal will apply equal force to the front and rear master cylindersJOhn
The Wookie said:
If the balance bar is set central (presumably what the OP means by zero) then the pedal will apply equal force to the front and rear master cylinders
Exactly what I mean - zero bias, both clevis are equidistant from the pedal. 
thanka for the legal tip too - I'll be locking it in place one adjusted both now and when the disk conversion happens, just want it available when I do the conversion and as I said earlier, at the moment it's purely a packaging exercise!
My biggest concern was if it's okay to put the residual pressure valves over onto the new MC's as without those the wheel cylinders will leak.
Just my 2p worth!
The original system - 2 circuits, you mention one for the front, one for the rear - is it not one for Front Right and Back Left, the other for Front Left Back Right? - Yes, I fully understand that if they have been modified this may no longer be the case!
Is the "thing" going to be used on the road? - if so, please stop reading ... . . if not, do you need a dual circuit? - a single, larger diameter master cylinder might be small enough to fit into the gap, and you would not have to have balance bars etc.
I am not a brake expert in the slightest! (well, until I slammed on brakes when I only had my front left wheel doing all the braking - then I very quickly became an expert at swearing - Back right was greasy, the other circuit had popped a seal in the master cylinder)
The original system - 2 circuits, you mention one for the front, one for the rear - is it not one for Front Right and Back Left, the other for Front Left Back Right? - Yes, I fully understand that if they have been modified this may no longer be the case!
Is the "thing" going to be used on the road? - if so, please stop reading ... . . if not, do you need a dual circuit? - a single, larger diameter master cylinder might be small enough to fit into the gap, and you would not have to have balance bars etc.
I am not a brake expert in the slightest! (well, until I slammed on brakes when I only had my front left wheel doing all the braking - then I very quickly became an expert at swearing - Back right was greasy, the other circuit had popped a seal in the master cylinder)
tapkaJohnD said:
Quote greenV8: Surely a rigid bar would apply equal travel rather than equal pressure. Both, surely? Why not equal pressure?
I'm visualising a setup where the 'balance bar' was welded solid so both master cylinders had equal movement.In this setup the displacement on both circuits will be equal. How much pressure that produced on each circuit would depend on the rest of the circuit, but it seems pretty unlikely that two completely different brake systems would have identical displacement/pressure characteristics.
GreenV8S said:
but it seems pretty unlikely that two completely different brake systems would have identical displacement/pressure characteristics.
My theory is that if you have 2 individual master cylinders with the same bore and throw as the original tandem master cylinder, and the same throw from the pedal (all of which is true on the parts I'm considering) the displacement/ pressure characteristics should be identical. Hydraulic theory that I've read supports this but I would like some confirmation that I'm not missing something!Peanut Gallery said:
Is the "thing" going to be used on the road? - if so, please stop reading ... . . if not, do you need a dual circuit? - a single, larger diameter master cylinder might be small enough to fit into the gap, and you would not have to have balance bars etc.
Not sure there's any situation, road or not, where I'd be happy with a single circuit brake system!Davi said:
My biggest concern was if it's okay to put the residual pressure valves over onto the new MC's as without those the wheel cylinders will leak.
Assume so but I don't have any experience with them! Worth finding out from someone who does as it's a recipe for drag if you get it wrong!Davi said:
GreenV8S said:
but it seems pretty unlikely that two completely different brake systems would have identical displacement/pressure characteristics.
My theory is that if you have 2 individual master cylinders with the same bore and throw as the original tandem master cylinder, and the same throw from the pedal (all of which is true on the parts I'm considering) the displacement/ pressure characteristics should be identical. Hydraulic theory that I've read supports this but I would like some confirmation that I'm not missing something!Davi said:
My theory is that if you have 2 individual master cylinders with the same bore and throw as the original tandem master cylinder, and the same throw from the pedal (all of which is true on the parts I'm considering) the displacement/ pressure characteristics should be identical. Hydraulic theory that I've read supports this but I would like some confirmation that I'm not missing something!
I don't know where that theory comes from. The amount of pressure resulting from a given displacement will depend on the rest of the system i.e. the area of the slave cylinder, and the force/deflection characteristics of the brake mechanism, pads, drums/rotors etc. Those valves in the rear circuit would obviously have an effect too. It seems vanishingly unlikely that the front and rear circuits and brake assemblies would have identical characteristics given that they have completely different designs.GreenV8S said:
I don't know where that theory comes from. The amount of pressure resulting from a given displacement will depend on the rest of the system i.e. the area of the slave cylinder, and the force/deflection characteristics of the brake mechanism, pads, drums/rotors etc. Those valves in the rear circuit would obviously have an effect too. It seems vanishingly unlikely that the front and rear circuits and brake assemblies would have identical characteristics given that they have completely different designs.
The rest of the system is exactly as it was - the only difference in the system at this point would be that is that instead of having a two master cylinder with 19.05mm bore, 25mm stroke in tandem, it would have two master cylinders with 19.05mm bore, 25mm stroke side by side. When it's later changed to disks, they are coming from a slightly later model and at manufacture the only change they made to the MC was to remove the residual pressure valves.The Wookie said:
Risk of dodgy side loads if you don't get it perfect and no guarantee of equal pressure as with different compliance characteristics for the same travel you could easily get a situation where one master cylinder is receiving more force than the other, hence more pressure
The actual manufacture of the pedal assembly is one thing I will have to be careful of, for sure. I'm using most of the original assembly, all I need to do to facilitate the change is shorten the bearing tube on the brake lever in the mill and reinsert the brass bush, the balance bar will then replace the clevisGassing Station | Suspension, Brakes & Tyres | Top of Page | What's New | My Stuff