Oiling o-rings and seals before installing?

[Robo McDuff]

I am rebuilding the master cylinder and air pack for the M51A2

Neither the rebuild kit nor the TM mention that you should oil anything before installing, but maybe that is standard knowledge or general information specified elsewhere. So should you, and is it a big problem if you don't?

Also, it looks like somebody deliberately put grease on the air-side of the piston plate or how you call it, maybe to lubricate it from that side and help keep away leakage of brake oil. Does that make sense or is common practice or big BS.

My problem is, I have to finish the air pack today, reinstall it, and the DOT 5 will come tomorrow afternoon. Also, the truck HAS to move tomorrow afternoon, so I cannot wait for the DOT 5 to oil rings and seals with. I do have some normal grass mower oil, or olive oil, or kitchen oil here, but nothing else.

 

[Robo McDuff]

TM ... 34-2-2 actually mentions that you should put a light film of brake oil in the slave cylinder and on the gasket and seals. Since it it mentioned there, for me, that means it is not necessary in other places?

 

[Jericho]

ALWAYS ALWAYS ALWAYS , nearly every O ring except in oxygen systems. Gaskets not so important, If noting else use the lubricating fluid in that system . Don't need a lot just enough to prevent pinching it

 

[Robo McDuff]

I know its better, but were are talking DOT 5 silicone brake fluid here. I know that a very small bit of different oil normally does not matter, but I don't know how DOT 5 reacts to other oils, especially what I have here: W90 gear oil, old engine oil, or kitchen oils, nothing else.

 

[frank8003]

Vaseline, motor oil, and any other fluid except what it is working in will swell o-rings up un-necessarily.
I have some "o-ring grease" here, commercial stuff specifically meant to swell up o-rings, that is for fixed static seal on vessals for NG and JP [almost everything JP] but it does not get used on our trucks.

That is for static o-rings that one will change the next time in there. Never reuse an o-ring.

The groove and the design is for either static or dynamic use as per design.
Absolutely do not "roll it in", if one must do that then it is wrong O-ring or something else is wrong.

How about a little history stuff.
Read this....................
View attachment o-ring packing NIELS A CHRISTENSEN 10021937.docx





View attachment O-Ring standard-size-o-rings.pdfView attachment O-ring and materials.pdf

 

[Robo McDuff]

Thanks, started to read, but its half hour after midnight here, bit too late to concentrate.

So what you are saying, don't use anything else than DOT 5 in this case.

What about the grease used by the previous owner; looks like a bad decision as well.

 

[gimpyrobb]

I use old dot5 fluid I save to lube the O-rings.

 

[Jericho]

Dot 5 has lubricity, use it as assembly fluid, Vasiline has little effect on quality o rings, we arnt talking about swimming in it ,,Apply in a very light coat. If you were to do a JOAP (oil analysis sample) all you would see would be an increase in SI ( silicone) an if you really swamped it a little frothing. Petrolatum ( maintenance quality Vasaline) , has been used for DECADES in jet engine assembly. I wouldn't introduce anything EXCEPT dot five in to a brake system, nor would I use Dot Five on a differetial as the lube . Little common sense goes along way ! I like Marvel Mystery Oil for most assy lube except new cam break in. It works every where else, BUT I never lube lug nuts with Kroil or WD 40 , learned the hard way a long time ago .