Master Cylinder Differences - 3 Port Vs 4 Port

[cunning plan]

Here are two Master Cylinders from JK:

One for a 1968 to 1970 (http://www.justkampers.com/vw-t2-bay-window-parts/t2-bay-braking-system-parts/brake-master-cylinders-wheel-cylinders/211-611-021-t-brake-master-cylinder-vw-t2-1969-1970.html" onclick="window.open(this.href);return false.

The other for a 1970 to 1979 (http://www.justkampers.com/vw-t2-bay-window-parts/t2-bay-braking-system-parts/brake-master-cylinders-wheel-cylinders/211-611-021-ae-brake-master-cylinder-non-servo-vw-t2-bay-1970-1979.html" onclick="window.open(this.href);return false.

They look identical, minus the 4th port (circled in green) on the '68-'70 version. I presume on a 4 port version, you can run 4 individual brake lines to each wheel, instead of one each to the front wheels, then a joint one for the rears?

Question: Will the later 4 port version fit in an Earlybay (1968) as they look very similar?

Question, part 2: If it cannot be fitted for some reason, can I take the Master Cylinder to an engineer to be professionally tapped out in the blank section to accept a 4th brake line?

Thanks guys!.. :shadey:

 

[vwed]

They might look similar but the internal bores are different in size which presumably has some implications for the hydraulics of the system.

 

[Moseley]

The ports you are referring to are for the brake pressure switches, not the brake lines themselves! At some point, around 1970/71 I thought, there was a move to two brake switches, presumably in case one failed, you have a backup to activate the lights. I believe they are wired in parallel anyway, so even if your van has two switches, you can run a MC with one port and leave a switch unplugged.

 

[cunning plan]

The ports you are referring to are for the brake pressure switches, not the brake lines themselves! At some point, around 1970/71 I thought, there was a move to two brake switches, presumably in case one failed, you have a backup to activate the lights. I believe they are wired in parallel anyway, so even if your van has two switches, you can run a MC with one port and leave a switch unplugged.

I have not got one in-front of me to test / look at, but why are they just for brake pressure switches - are the threads a different size?

I am thinking that the switch works using pressure applied to the sensor part which is screwed into the M/C. Pressure, is pressure, so theoretically, you could place the switch in any of the 'ports' and it would work. So, what prevents you from moving the switch further down the system and using all 4 'ports' as brake lines?

This thread (http://forum.earlybay.com/viewtopic.php?f=4&t=37060&start=0" onclick="window.open(this.href);return false made me consider it as on RHD Earlybays (which mine is, but I cannot remember the exact brake setup as it has been so long), the switch is moved back down the system to a T-Piece as there is no sufficient room for the switch to be used on the M/C as on LHD due to the chassis design.

Logically, this makes you think if a brake switch fits into a standard brake line T-Piece, it must be the same size as a brake pipe, meaning, the 'ports' in the M/C must also be the same size and capable of pushing pressure out.

So, logically, it would work, right?

:?

 

[Moseley]

Whilst I agree with the pressure is pressure statement, I think you'll find that the female fittings are different. I am saying this from memory, so I stand to be corrected, but the pressure switches are parallel threads and sealed with an O-ring against a face. The brake lines are sealed with the flared end of the copper which I'd imagine is different.

Ours was a RHD and there was no problem with the switch location. I'm struggling to see what you're gaining by running 4 separate lines though - even if one line springs a leak, you'll still lose the braking effort on two wheels. Plus, with twice as much copper pipe, as the fluid degrades or air gets into the system, you're going to end up with even more pedal travel than if the lines were tee'd.

 

[admwllms]

I just fitted this one to my 1968 bus a couple of weeks ago mate, bought two new switches too. Went in fine and no issues, hope that helps. £104 quid from Cool Air

http://www.coolairvw.co.uk/Item/Shop_by_Vehicle~Baywindow_Products~Baywindow_Brake_Products~Master_Cylinders~Master_Cylinders/211611021Q/T2_68-69_Master_Cylinder_LHD.html" onclick="window.open(this.href);return false;

 

[cunning plan]

Whilst I agree with the pressure is pressure statement, I think you'll find that the female fittings are different. I am saying this from memory, so I stand to be corrected, but the pressure switches are parallel threads and sealed with an O-ring against a face. The brake lines are sealed with the flared end of the copper which I'd imagine is different.

Ah right, this makes sense!

Ours was a RHD and there was no problem with the switch location. I'm struggling to see what you're gaining by running 4 separate lines though - even if one line springs a leak, you'll still lose the braking effort on two wheels. Plus, with twice as much copper pipe, as the fluid degrades or air gets into the system, you're going to end up with even more pedal travel than if the lines were tee'd.

Well the thinking was, similar to the way in which front brakes are set up with separate brake pipes, if one blows, you 'should' still have enough braking force left in one wheel to bring you to a stop to investigate, until all of the fluid from the reservoir is depleted and you start pumping air in.

With the rear, if the pipe is damaged for whatever reason half-way down the bus, that is both rear brakes out of action, so I was trying to think of a way to run x4 individual lines.

To clarify you thoughts though, does that mean that instead of two individual circuits at the front, and one at the rear, making 3 circuits, that actually there is only 2 circuits of pressured brake line, front and back. Then, even if the front is split into two circuits, if one line blows, the other will only be effective for a few pumps of the brake peddle as technically it is the same circuit? :?:

I just fitted this one to my 1968 bus a couple of weeks ago mate, bought two new switches too. Went in fine and no issues, hope that helps. £104 quid from Cool Air

http://www.coolairvw.co.uk/Item/Shop_by_Vehicle~Baywindow_Products~Baywindow_Brake_Products~Master_Cylinders~Master_Cylinders/211611021Q/T2_68-69_Master_Cylinder_LHD.html" onclick="window.open(this.href);return false;

Picture helps, thanks buddy

 

[Moseley]

Regardless of whether you have one brake line that later on splits into two, or have two separate lines coming from the master cylinder; if a pipe splits, you will instantly lose all braking force to two out of your four wheels. The reason being, there are only two effective pistons in the MC - when you push the brakes, the fluid coming from either piston will always take preference to the path of least resistance. So as soon as you burst a hose, the piston that controls that half of the braking circuit will just pump fluid out of the hole, and the only thing that stops your pedal hitting the floor is the fact that the other piston will still have the resistance in the other half of the circuit.

Does that make sense?!

 

[cunning plan]

Yes, got it: dual circuit = two pistons, which can be split into 100000 brake pipes, but there is still only two 'active circuits' of pressure.

If a pipe fails anywhere in one of the two circuits, the piston for that circuit will just pump the pressure out of the failed line.

Cheers buddy, it is always good to talk about the way things work and the theory behind them arty0047: