Which is correct

[marcusclive]

Evening gents,
not been on here for a couple of years but the car is now nearing completion and I need a bit of expert knowledge.
On the PDW valve are the three forward most outlets supposed to be larger dia than the back two.
the original brake pipes obviously fitted but the new ones don't and I would like to know which ones are / were correct.
marcus.

[Staggard]

Evening gents,
not been on here for a couple of years but the car is now nearing completion and I need a bit of expert knowledge.
On the PDW valve are the three forward most outlets supposed to be larger dia than the back two.
the original brake pipes obviously fitted but the new ones don't and I would like to know which ones are / were correct.
marcus.

I thought the PDWA valve connections were all the same size 3/8 unf 24 tpi could be wrong, at least it will bump the post back up to the top.

[Mark Shutt]

those 3 at the front are 7/16th unf, I don't know why they used bigger as the pipes are all the same size!

Mark.

[Westfaling]

It is a bit odd that they used smaller fittings with the same size pipes but based on my car the smaller two are for the rear brakes.
Nick
PWDA.jpg

[marcusclive]

Many thanks for that gents, now do I bother sending my new brake pipes back or not.

[V Mad]

I would guess they fitted two different sizes to avoid wrong connections during manufacture. The pipes would have been preformed unlike replacement ones nowadays.

[Mark Shutt]

Many thanks for that gents, now do I bother sending my new brake pipes back or not.

yes, they have sent you the wrong products.. rear ones are smaller bore to bias brake pressure to the front I'm guessing.

[Paul Roberts]

yes, they have sent you the wrong products.. rear ones are smaller bore to bias brake pressure to the front I'm guessing.

On the right lines there Mark.

The pressure will be equal but, if one pipe is a larger ID, the volume of fluid flow will be greater in the larger pipe. This may well be the requirement, here, as the front disk calipers will have a greater volume of fluid to movement ratio than the rear drum brake cylinders.

Just don't ask me what the respective sizes are as, sadly, I have no idea.

[Mark Shutt]

if you look down the holes in the PDW valve you can see a difference in sizes for front and rear, I reckon they could have still got away with using the same unions as the holes aren't much different in size though but then again I didn't design the set up so only speculating!!

[Paul Roberts]

You're spot on, Mark, that would effect the same outcome regardless of brake pipe diameter; as the smaller hole would be a choke.

[milothedog]

Is that the case? the pipe I D playing a part in it ?, To my way of thinking and what I was taught, it's only the greater area of the surface that the fluid or air (in the case of air brakes) can act on that will generate the increase in force? As I understand it the smaller ID only allows a quicker response (not as much to move)

Normally if a bias is required /designed in, and it's not achieved within the operation of the Master Cylinder then an additional device is installed in the system to either vary or step down the pressure proportionally .

[HersnotHis]

Is that the case? the pipe I D playing a part in it ?, To my way of thinking and what I was taught, it's only the greater area of the surface that the fluid or air (in the case of air brakes) can act on that will generate the increase in force? As I understand it the smaller ID only allows a quicker response (not as much to move)

If I can put my "clever boy" hat on again for a moment can I say you are perfectly correct - exactly as I was taught also!

[Paul Roberts]

It's dependant upon closed circuit or free flow, Ian.

The master cylinder will determine the overall pressure generated (in a closed circuit) or volume discharged (free flow) and the fluid will, nominally, flow more readily through a larger pipe than a smaller one. Because there is equal resistance to pressure, at the operating (slave) end due to closed circuit, the greater volume of fluid will still tend through the larger pipe albeit at the same pressure as the smaller pipe. (For the purpose of this point of discussion "friction" within the pipes is practically irrelevant.)

The working "applied" pressure at the respective cylinder/calliper piston is a function of circuit pressure x surface area of the respective pistons.

Of interest; if there were too greater bore difference between the circuit pipe work (think stupidly tiny bore or choke hole) the lack of fluid flow would create a temporary pressure imbalance within the circuit that would cause the shuttle to move across and bring on the warning light. If the pressure were built up slowly enough the shuttle would remain central.

If the pipes were disconnected from the slave end (free flow) there would be only frictional resistance and all other things being equal; the bulk of the fluid would tend to discharge through the larger pipe.

[HersnotHis]

It's dependant upon closed circuit or free flow, Ian.

Surely we are talking about a closed circuit car braking system only here! As Ian says the resultant is dependant on the area to which the pressure is applied.

For the purpose of this point of discussion "friction" within the pipes is practically irrelevant.

The point is not friction but circulaminar flow which is significant in brake pipe type bores. However what is forgotten is that in a braking circuit (or most closed hydraulic system) the actual fluid flow is minimal

H

[Staggard]

The poor fella only wanted to know where the bigger unions went Have you ever wished that you never bumped a post