Once you've spent all that time re-building your pump you might want to test to see if it is working as designed. Alternatively you might want to test a vacuum pump before you decide to reseal it; you might want to do a before and after rebuild test. This procedure is given in chapter 43-660 in the FSM.



First thing to consider is the effect of absolute pressure. Where I live just a few metres above sea level the vacuum pump is able to “pull” or “suck” a greater pressure difference. With good seals a vacuum pump might be able to create a negative pressure close to 1 bar. At higher altitudes the vacuum pressure achieved on the gauge might not look so good. This doesn't necessary mean that your pump isn't working efficiently. (See LINK for more information)



For this reason the FSM has specified that at an idle speed of 750 rpm the vacuum pump should achieve a 0.5 bar pressure (which I assume should be possible at most altitudes) within so many seconds when connected to the brake booster / and or the rest of the “comfort” vacuum system. To do this test you are meant to make up a little T section tube that you fit between the brake booster and the check valve. A link for making one of these bits for yourself is here LINK. Chapter 43-660 gives the piston vacuum pump connected to the brake booster 7 to 9 seconds at 750 rpm to reach 0.5 bar. The system is not meant to leak more than 0.2 bar in 30 seconds – it specifically says that a leak of 0.2 bar in 30 seconds is permitted; so there's a subtle difference that probably is a bit misconstrued in the translation from the German version (sorry I can't be bothered to check – I'd have to start Windows to see my German copy of the FSM...)



The reason why Mercedes decided to provide test specifications for the vacuum pump when it is fitted to the brake booster and not just when there's a gauge fitted to the end of the pump is probably because the pump would evacuate the connection so quickly it would be difficult to measure. At 750 rpm you've got 12.5 cycles (or strokes of the piston) per second. As the pump should evacuate the volume of the brake booster and the line to the pump to 0.5 bar within 7 to 9 seconds that's 87.5 strokes for that volume if 7 seconds is achieved.



The displacement volume of the OM61X piston vacuum pump is 7.85 cm3 (see below). Piston pumps are generally expected to have an efficiency of 85% to 90% so the pump should be able to shift about 6.67 cm3 per stroke or 83.4 cm3 per second at 750 rpm (taken with 85% efficiency). If anyone has a flow meter and wants to check that for me I'd be happy to hear from you!



Another reason for adding this information is that people sometimes ask whether they can replace the mechanical pumps with electric ones – this estimated flow rate should give people a ball park idea of what is expected from the vacuum system on a W123 / W126 / W115 / W116 (although the W115 and W116 probably don't have the piston vacuum pumps fitted – only the diaphragm ones).

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!