Сначала общее название:

Английский автомобильный словарь
...
grease fitting пресс-маслёнка, тавотница
http://spidometrs.ru/content/view/181/25/1/6/

Теперь о "зерке":

Grease Gun Buyers Guide
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In 1924, the Allyne-Zerk Company of Cleveland, Ohio was purchased by Alemite, and the Zerk line of lubrication fittings and hand grease guns was added to the Alemite line. The Zerk design, named after Oscar Zerk, used a fitting much smaller than the Alemite pin-type and did not lock the hose coupler or hand gun and fitting together. Instead, the seal between them was maintained by the pressure of a pushing action when the operator applied the coupler to the fitting. This became known as a *****push-type system****.

http://www.machinerylubrication.com/article_detail.asp?artic...

Вот и выяснили отличие: это "push-type system"

МТ:

push-type lubricating fitting пресс-маслёнка
push-type lubricating fitting смазочный фитинг нажимного типа
http://www.multitran.ru/c/m.exe?a=phr&s=push-type&sc=28&l1=1...

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Note added at 1 час (2007-12-20 17:30:10 GMT)
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Вместо термина "масленка" можно использовать тот же "клапан" или "фитинг".