Clutch Vacuum Booster

Well, I decided to experiment with a clutch booster. I find the 348 (twin) clutch rather heavy and, just for my comfort, I installed a small remote PBR VH44 brake booster which comes with a 19 mm master cylinder, same diameter as the 348 clutch master. This booster version is offered by an Australian company (seller 1947dgb on Ebay) which reconditions PBR remote boosters and also makes the special 19 mm version. The standard versions have 5/8 master cylinders which would probably run out of travel if combined with the 19 mm 348 clutch master.

Just tested the installation. First, without vacuum - the clutch operated exactly the same as before. With the vacuum connected, it was much lighter but I could still feel the "peak" when depressing the pedal. Perfect, nice and light but not boosted so much that you lose the feel. Tested the clutch with the rear end still on axle stands, yet to see how it works when I drive the car but I don't expect any problem.

The hydraulic plumbing was simple - just bought another 348 clutch hose that goes to the slave cylinder and a few adapters to connect to the PBR master cylinder. The installation can easily be reversed.
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PBR VH44 I bought has 3/8 x 24NF ports and the 348 clutch hose has M12 x 1.0 connectors. I could not find adaptors 3/8 to M12 so I bought 3/8 x 24NF to M10 x 1.0 and M10 x 1.0 to M12 x 1.0. The seat inside the female M12 was inverted (see

 

There was a sudden self-post. Again:

PBR VH44 I bought has 3/8 x 24NF ports and the 348 clutch hose has M12 x 1.0 connectors. I could not find adaptors 3/8 to M12 so I bought 3/8 x 24NF to M10 x 1.0 and M10 x 1.0 to M12 x 1.0. The seat inside the female M12 was inverted (see the drawings below) so I had to machine it to the inward flare. I got the adaptors from: http://www.ebay.com/itm/Brake-Line-Thread-Adapter-Male-3-8-x-24-Inverted-Female-M10-x-1-Inverted/332040601014?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649 and http://www.ebay.com/itm/Brake-Line-Thread-Adapter-Male-M10-x-1-Inverted-Female-M12-x-1-Inverted/331631059619?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649.
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LE10117 has 5/8" cylinder bore, smaller than the 348's 3/4 clutch master. So, to displace the volume of fluid sent by the 3/4 clutch master, the booster's piston has to travel more than the 348's clutch master piston to disengage the clutch. With this combination, the booster's piston may run out of travel and you will suddenly have hard clutch pedal near the bottom of its travel. Also, with a smaller diameter piston, the booster may overboost (too light pedal) and you lose the feel. Of course, one can try with the 5/8" booster, it may still have sufficient piston travel. The booster I installed gave me a perfect combination of light clutch and still good feel. It has been working great driving the car (I think I drive my 348 more often since the mod). There is only one strange behaviour: if I depress the clutch before starting the engine then, when the engine is running (vacuum present) and I release the pedal (gear in neutral), the pedal will move up with some delay. Afterwords, it works fine. I think this is because the vacuum is applied post booster diaphragm movement to full travel. What I currently do is start the engine without depressing the clutch and everything is fine. I will soon add an inline solenoid valve to the vacuum line so that I can control (via a timer relay for the solenoid valve) when to apply the vacuum - after I have started the engine with the clutch depressed and after I release the pedal.

 

If you want to try with a (cheaper) Lockheed booster, then it is probably better to go with the LE72696 as it has 11/16 cylinder bore (closer to the 348's 3/4) and it has 1.9:1 ratio which is the same as my PBR VH44. This ratio is perfect for the clutch. With the ratio of 3:1 you mentioned, the clutch will probably be too light. Otherwise, check the diameter of the canister; my PBR is 7.5" and it fits fairly tight in there. And I had to trim the inner plastic cover of the rear lamp in order to be able to get it in and out.

 

That's good news. A contingency I thought about (and bought) is a vacuum tank, as on the picture. I thought the booster may run out of vacuum when shifting often and at higher throttle openings (lower vacuum) but, so far I did not need it at all and did not install it. I also bought a Comp Cams electric vacuum pump which can be used as an option, instead of plumbing-in the vacuum line to the plenums. Image Unavailable, Please Login Image Unavailable, Please Login

 

It has been working great on my 348. I did not need to re-bleed the system since the installation more than 3 years ago. There was no need to add either the vac tank or the vac pump.