Power Brakes on PMO carb manifolds?

[yundar]

Hi all... Doing some thread searching on using carburetors on a 78 SC with power brakes. I find an old thread about an ATE check valve that works with PMO carb manifolds specifically for this purpose. Does anyone know what the part number for this is or where to find one?
Thanks, Andy.

[Flat6pac]

Call PMO, Best bet
Bruce

[yundar]

I will try to get ahold of him. Just looked at the manifolds and I didn’t see a port on them so it must have to come off the carbs which are Zeniths.

[yundar]

“Just talked to Richard (at PMO), and he said all I need is to install a $10 ATE check valve, which he's sending me. No need for any further drilling or tapping, the one vacuum source will do it, with the check valve. I assume that what the check valve does is hodl a certain amount of vacuum in the brake booster, which is then augmented by the manifold source. Or something...

Stephan“

This is a copy of the text from the thread I was referring too. It’s dated 2001 so I’m sure it’s quite done. I’m not seeing the port he is referring to on the manifold itself? There is a larger single port on the carb itself. One on either carb and about 4-5mm in dia.

[juanbenae]

you need to tap a hole in the manifold to get any decent vac for the brake assist.

[Ayles]

I went with a small high end vacuum pump for electric cars. From a company called Metric Mind. Don't confuse this with pumps out of the Summit catalog this one is Swiss made, small and quiet. Been working great the last couple years.

[yundar]

Cool... Good ideas! I have seen the thread with multiple holes (four I believe) drilled into the manifolds. I wonder if using this mystery check valve allows you to get away with only one hole?

[john walker's workshop]

With only one vacuum port, you will likely get less assist after pressing the pedal several times in a very short period. I used a sportomatic vacuum chamber once, which worked well.

[yundar]

Is the Sportomatic vacuum chamber like a stand alone vacuum pump? I have been reading more and am now considering a stand alone 12v vacuum pump and not using engine vacuum at all.

[john walker's workshop]

It's just a flat can that holds vacuum.

[RDM]

Originally Posted by john walker's workshop

It's just a flat can that holds vacuum.

Sort of like a vacuum reservoir that stores up vacuum over time in case you need a lot of it in a short period.

[KTL]

Both manifolds have a round section of aluminum projecting from the middle cylinder runner. The location we typically use is on the left side manifold of the engine and it's very easy to drill a hole into it and tap it for a large barb fitting. Obviously you want to drill it to be sized for the threads on the barb fitting. 1/2 in. inside diameter hose barb fittings are typically 1/4 in. NPT. I used a 1/4 in. -18 thread per inch NPT tap to do mine.

You can see the hole in this picture



Here's another picture of the right side manifold (before I stripped off the METALLIC BLOO powdercoating) with the un-tapped projection. There's a dimple in the spot you need to drill so it's easy to drill exactly where you should



It's strange how some guys have found the vacuum to be insufficient on cars that did not originally have vacuum-assisted power brakes installed from the factory. Two of my friends have had a 1970 and 1976 racecar and they didn't get much brake assist from the vacuum port hooked up to a factory brake booster mounted in the front trunk. Maybe it was because they had a long length of rubber hose that collapsed under vacuum? Whereas the factory installation is a long section of thick, reinforced hose from the upper area of the engine bay, down along the firewall/rear seat area, into a metal tube at the center tunnel. That metal tube runs the entire length of the tunnel and goes up into the front trunk before another short section of hose is used to connect onto the booster nipple.

The guy with the 1970 had the booster and master cylinder assembly nicely installed in the trunk (he did a G50 trans installation and needed the pedals for the hydraulic clutch) but found that he didn't notice any difference in the brakes. Maybe the used donor vacuum booster he bought was failed? The other guy with the 1976 just installed a switchable electric vacuum pump and a PVC pipe reservoir in the trunk. Worked great and he installed a vacuum gauge so he always knew how much "brake help" he had in the reservoir just by looking at the gauge on the dash.

In my own case, I found the vacuum port on the PMO manifold (with the check valve) worked great in my '79 SC racecar. Seems to pull enough vacuum all the time to allow the original brake booster to provide plenty of assist. I never found my brakes to be too much effort (they're larger brakes, along with a 930 master cylinder) and i'm not exactly a big strong dewd- 6' tall, 175 lbs and kinda thin.

[A 911 Dream]

Ayles, do you mind posting the manufacturer details of your pump?

Sounds like a great solution.

Chris

[Ayles]

No problem!

Here is a link to the place I ordered from:
EV Vacuum pumps | Metric Mind Corporation

I purchased mine back in 2013 and it looks like they are only offering one model now. The one I have is Model 70/6E. I paid around $300 all in with connector kit and shipping.

[frankc]

The tapped middle port on my PMO manifold with a check valve has been working great as a vacuum source for the power brake booster on my '77 for over 30 years now.