Brake plumbing question on '68 hotrod

[MikeDL]

Now that the garage has warmed up, I'm back out in it with my project car. First thing this year are the brakes. My '68 was a track car and has had a later front suspension with A calipers on it.

When I discovered that the front brake lines were lost at the paint shop, I also discovered that the original brake plumbing was replaced to support a cockpit mounted proportioning valve, presumably to better balance the bigger A calipers with the original rear M calipers. I have to decide if I should keep the valve or return to the stock arrangement.

If I remove the valve, the line split fittings for the front master cylinder are all NLA -- so with that route, I'll have to come up with something nonstandard.

If I keep the valve, I need to worry about if it's still in good condition but getting new hard lines shouldn't be too difficult.

Any insight on how important the valve is here? The car won't be tracked.

[ttweed]

Originally Posted by MikeDL

When I discovered that the front brake lines were lost at the paint shop, I also discovered that the original brake plumbing was replaced to support a cockpit mounted proportioning valve, presumably to better balance the bigger A calipers with the original rear M calipers. I have to decide if I should keep the valve or return to the stock arrangement.

Remove the proportioning valve. There is no reason to limit the pressure of the rear brake circuit with that setup. The A calipers are essentially the same as the alloy S calipers that were used on the '69 and later 911S (in terms of piston and pad area), and they were paired with the rear M calipers with no proportioning valve by the factory. Limiting the pressure to the rear brake circuit is not necessary, as the difference in piston and pad areas already insures proper front brake bias. Limiting the rear pressure with a proportioning valve will only reduce the overall braking effectiveness of the car. The only time a bias valve is necessary is if the rear brakes are capable of locking the rears up before the fronts lock.

It is not entirely clear from what you wrote, but if you are saying that someone installed a proportioning valve on the front brake circuit, you should definitely remove it. That is a very dangerous setup and not recommended in any situation.

TT

[Grady Clay]

OK,
Lets get a clearer picture.

Front:

Has ’69 and later struts, A-arms, cross member and steering track.

The struts have 3.5” spacing for the A-caliper brakes.

The steel brake pipes are there but are “split’ somewhere?

What master cylinder is in place?

Not two master cylinder set-up?

Original pedal assembly?

Is the single brake pipe in place to the rear? Any modification?


Rear:

Still original SWB?

What brakes? M-caliper? Vented?

Brake pipes and hoses original?



What is your intended use?
What level of preparation? (Budget?)
Time-line?

Best,
Grady

[MikeDL]

Thanks guys -

'69 and later suspension (struts, a-arms, crossmember, steering rack). 3.5" spacing, A calipers. A single dual circuit master cylinder. Original pedal assembly as far as I can tell. Rear brakes are original SWB with non-vented discs and M calipers (the car is a Euro T).

From what I can see, the rear brake line from master cylinder to calipers is exactly as stock.

The front, however has a single line leaving the master cylinder, going into the tunnel where it hooks into an adjustable valve, with the adjustment reachable from in front of the shifter. A line leaves the valve and goes back to the front until it exits the tunnel and goes into a T juncture. The lines from the T to the two front calipers are both missing.

As for intended use -- I'm not trying to return it to original, don't intend to track it again --just want it to be safe and work well. All calipers have already been rebuilt by PMB, and the master cylinder will be replaced. Budget and timeline aren't factors.

I'm suspicious of the valve and would happily remove it, but I wanted to make sure it wasn't necessary to compensate for the increased braking power of the later front calipers. The only complication to that approach is that the socket piece and banjo bolt shown in the PET (illust. 604-05) are NLA.

[Grady Clay]

Throw away that valve and re-plumb as original.

Question:
Are there two hoses from a dual-section brake fluid reservoir?

Be sure to replace all four flex hoses with OEM original (not SS).

Inspect the pipe at every screw connector. If there any ‘twist’ to the pipe, replace the pipe and screw fitting.

Best,
Grady

[MikeDL]

Yes, two lines from the reservoir. I already have new OEM flex hoses, and will look carefully at the hard line junctions. I'm going to search for the NLA front line banjo bolt -- maybe I'll get lucky at a dismantler.

Thanks for your advice!

Regards,
mike.