Master Cylinder Question

[mrdew]

I pulled this off of the master cylinder. I am rebuilding it, because I was unable to get the brakes to hold pressure, they would build it but not hold it. But in the process of tearing it apart, I found this. It has the rear brake line connected to it.

[thaddeussmith]

what exactly is your question? All i see are statements of fact...

[mrdew]

ahh sorry I got distracted. What is it, and what is it's normal operation

[thaddeussmith]

lol. not sure why it is different, but I've seen it on all of the MC's I've come across. Maybe it has something to do with pressurizing the line since it's the only one going to the rear? Someone will chime in with the technical data, I'm sure.

[TKMad]

That is a residual pressure valve for the rear brakes. It keeps a little pressure in the lines for the rears only.

[TobyB]

...and is not present on any of the cars I have owned or parted.

But I tend to do early cars.

And the BMW rear system does not particularly require it.

t

[02for2]

I believe it's an add-on. My 1976 02 doesn't have it and it was bone stock when I got it.

Cheers!

[M3M3]

all four 2002's of my have one of those on rear brakes.

73,74,75,76. not an add on.

[TobyB]

Well, that's funny- Marshall, are you sure you're not makin' that shit up?

Well, sumbeech, lookee here-

#04 PRE-PRESSURE VALVE 1 34314650332 $21.70

right on realoem, so it must be right! (another grin)

Wonder if I've just been missing it all these years (possible)

or if ALL of them were lost by previous owners replacing the masters?

That's HY- larry- ous.

ya learn yer an ijjit at least once a day.

t

[M3M3]

Well, that's funny- Marshall, are you sure you're not makin' that shit up?

Well, sumbeech, lookee here-

#04 PRE-PRESSURE VALVE 1 34314650332 $21.70

right on realoem, so it must be right! (another grin)

Wonder if I've just been missing it all these years (possible)

or if ALL of them were lost by previous owners replacing the masters?

That's HY- larry- ous.

ya learn yer an ijjit at least once a day.

t

LOL! it appears seattle cars and dc cars were made in a different factory... seattle cars had no brake press valve and used oil filter cartridges instead of spin on's! clearly i must avoid making generalizations when refering to 2002's! or just stop makin' shieet up! :-)

M

[Cameron]

It helps equalize front to rear brake 'timing' The wheel cylinders have to overcome the spring pressure before contacting the shoes, while the calipers simply slide into contact. Leave it in unless you are converting to rear disc or very bad things will happen

[TobyB]

errr, bullshit. I've never run a car WITH one, and unless you consider

me on public roads to be a very bad thing

(which I could fully understand)

nothing bad has ever happened for not having it.

Ran the race car the same way.

Thing is, the rear wheel cylinders now have springs in them to

preload them against the shoes, and the shoes are held against

the drums by the adjusters, so all the slop's mechanically preloaded

out of the system already. And from what I've found, you really don't

want the shoes touching the drums much (at the track) or the rear

brakes start to bind up.

So maybe this thing was for rear wheel cylinders without springs?

huh.

t

[mrdew]

I think I may just ditch it. The entire brake system is brand new, including rear disc brakes. I do have a new proportioning valve in place also.

[jgerock]

Swinging into another corner of the German automotive front...

Sorry to bring the ol' VW into this, but some VW master cylinders (dual circuit type) have restrictor drillings in them and some have external residual pressure valves to help balance the front to rear bias.

Per the Bentley VW Manual for '70 and later Beetles and Karmann Ghias, cars with front disc brakes have a master cylinder with restriction drillings instead of residual pressure valves. These drillings are 3.5mm in diameter. Except for 1973 and later models, all cars with four-wheel drum brakes have residual pressure valves between the master cylinder outlets and the pressure lines (they look like hexagonal fittings). The master cylinders for 1973 and later models with four-wheel drum brakes also have restriction drilllings rather than residual pressure valves. These master cylinders must not be interchanged with those used on cars with disc brakes.

[M3M3]

I think I may just ditch it. The entire brake system is brand new, including rear disc

definitely ditch it with rear disks. they are "self adjusting" and you do not want any residual pressure in the line.