Rear Brakes - What is the purpose and function of this?

[PeterAllen]

 

 

And, how does it relate/interact with this?

 

 

Thanks.

[dat2kman]

Bottom pic is the thingy that lights a light on the dashcluster, when either the front or rear circuit has a leak or is faulty, it has a piston inside that slides back/forth that pushes up the switch in the middle.

Near it will also be a seperate rear brake line pressure reducing calve which stops drum brakes from locking.

 

The middle pic is interesting, barring the timber supports, the rear trailing arms and brake backings dont look like a Z

If it is a disc braked rear end something, the little square inline fitting could be some sort of anti brakepad knock back, or residual line pressure valve.

 

Top pic that circled item, dunno, rest looks normal Datsun, residual line pressure thing, to keep rear drum shoes  in close proximity the drums, so you dont have massive pedal drop when first applying brakes!

[PeterAllen]

The circled item and item in the forefront in the second photo are one and the same!

[.]

Obviously there was something wrong with that whole brake setup. Otherwise how did that car get tangled up in the rafters? Man that must have been some accident :-D

 

[Scoota G]

The circled item look like a proportioning valve which lowers the pressure to the rears and stops them locking first. Common with disc brake conversion setups. I need to fit one on my V8 Zed.

[nat0_240_chevZ]

circled is the residual pressur device for the rear brakes.

I have had this located in some of mine and my mates cars up in the engine bay, next to the pressure diferential switch,

I feel it was moved to due to bleeding issues or otherwise pressure drop issues for the rear brakes, At this stage I am unable to work out if this was moved rearward or frontward and when.

Some master cylinders had a rear resid pressure valve for the rears in them, some didnt, when some people have attempted mix n match with new masters they have found there rear brakes drag, due to having 2 residual presure valves in the system, and other have none, causing rear shoe knock back,so have a spongy pedal due to this.

 

Like most things in manufacturing, its all little changes along the way, never assume the first car and the last car on/off the production line within a week or fortnight period are "exactly" the same.

Nat0

[Six_Shooter]

It's not a residual pressure valve, it is however the early rear proportioning valve.

 

In 1973, this was replaced by a different type of prop valve that used the right front brake pressure to modulate the rear pressure, and was installed on the firewall.

[Scottz]

Yep - First 2 pics -  Rear proportioning valve  or  Pressure Regulating Valve (same thing) for 240Z, only 1 inlet and 1 outlet. Found at rear end exactly where first pic has it circled Keeps things in proportion, but not necessarily equal  That is - It prevents rear brakes locking under heavy braking.

For 260Z it is found up front on the firewall and has 2 inlets and 2 outlets (1 each for front and rear brakes).

Last pic - Dual Circuit Pressure Differential Switch. Found in both 240Z and 260Z close to the master cylinder. This device monitors the pressure in the 2 seperate curcuits (Front & Rear). If you get a leak in either front or rear circuit the switch closes and illuminates a warning light.

[PeterAllen]

From a bit of reading I've got the impression that it is not a proportioning valve but more of a tiny reservoir device which creates a time delay in transmitting fluid to the rear brake. When the brakes are applied fluid is immediate pumped into the front system while there is a tiny delay to the rear brake while the reservoir is filled, thus preventing the rears from locking up first. Once the reservoir is filled the same pressure is applied to the rears as the fronts. Anyway, that's how I've interpreted the info.

 

 

[red76z]

Should this valve be disconnected if putting r31 rear calipers an a manual proportional valve in line?

[RB30X]

Peter, if there is no compression of brake fluid and the system is sealed, how can this reservoir delay the rear brake application if it can't drain down. I would've though the system would always be charged. Maybe it regulates flow?

 

 

[Scoota G]

Should this valve be disconnected if putting r31 rear calipers an a manual proportional valve in line?

Replaced with the manual one would be the go.

[PeterAllen]

Peter, if there is no compression of brake fluid and the system is sealed, how can this reservoir delay the rear brake application if it can't drain down. I would've though the system would always be charged. Maybe it regulates flow?

 

Tim – I had imagined a diaphragm/bladder of some kind.

 

HOWEVER:

 

Someone sent me an extract from a 240Z service manual (I only have a 260Z service manual) and the item is described as a PROPORTIONING VALVE. However, it functions only as a proportioning valve once a certain 'split' pressure is reached, i.e. only under heavy braking do the fronts receive a higher pressure.

 

red76z - Yes, definitely remove/bypass it if you are fitting a Wilwood, or other, proportioning valve.

 

Here is my new brake plumbing in that area: