master cylinder check list

[siteunseen]

I think I'll try this tonight.  I can't remember doing one before but that doesn't mean much, memory...bad, very bad.

Put rags everywhere then take the old one off and clean up the booster. 

Bench bleed the new one then put the plugs back in the bottom ports

Bolt it onto the booster

Connect the brake lines

Bleed all the old fluid out

Thanks for helpful advice.  Cliff 

[siteunseen]

This turned out to be easy so far.  The tubes in their bench bleed kit aren't made for bottom ports, too short.  I've got a syringe and  will just do it backwards.  Thanks for that tip, somebody? 

Chris @grannyknot  you were right, I needed a new one.  Thanks for that suggestion.

[siteunseen]

Anybody know the sealer between the master cylinder and it's backing plate?

Cleaning the backing plate I see it had some sort of sealer where it slides up to the M/C, silicone I think, it was still rubbery when I scraped it off.  Just some type RTV?  I have a tube of the black oil resistant. 

Thanks

Another question.  Would some white lithium grease be okay on the plunger out of the booster into the M/C?  Like I use on the door hinges?

 

Edited February 16, 2017 by siteunseen

[JSM]

In the few I've done I've never seen any sealant that I can recall. I just did my power booster and there was nothing. Maybe the PO dabbed some on just in case it weeped interernally?

I've never lubbed the plunger. It should be pretty dry in there. 

[heyitsrama]

Raybestos has a brake caliper lube that I use. I think I've done something like 4-5 cars motorcycles in total. Nice product.

[siteunseen]

After a lot of reading I found a pretty interesting thread that proved true reading about other cars.  Sealant between m/c and the backing plate, I used rtv ultra.  No seal between the backing plate and the booster. The booster actually draws air from a small groove milled into the booster.  It runs from the center hole to the 11 o'clock spot just pass the backing plate.  I coated the push rod only with lithium grease.  It slid into the m/c nice and smooth.  The booster hole is clean and dry, another reason not to use sealant, leaky m/c can leak so you can see it's leaking.

[JSM]

27 minutes ago, siteunseen said:

After a lot of reading I found a pretty interesting thread that proved true reading about other cars.  Sealant between m/c and the backing plate, I used rtv ultra.  No seal between the backing plate and the booster. The booster actually draws air from a small groove milled into the booster.  It runs from the center hole to the 11 o'clock spot just pass the backing plate.  I coated the push rod only with lithium grease.  It slid into the m/c nice and smooth.  The booster hole is clean and dry, another reason not to use sealant, leaky m/c can leak so you can see it's leaking.

What was the logic on the sealant?

[siteunseen]

Nissan did it. 

[JSM]

2 hours ago, siteunseen said:

Nissan did it. 

 That's fine, but what is the purpose? If air is needed to be drawn in, the sealing does what? It's still open to air. Maybe it helps with control of the needed air?

[siteunseen]

I put sealant on the opposite side of the backing plate, where it mates with the master cylinder.  

[Patcon]

4 hours ago, siteunseen said:

I put sealant on the opposite side of the backing plate, where it mates with the master cylinder.  

cliff,

I think he is asking what is the logic to sealing one side of the plate to the master cylinder and not sealing it to the booster? I don't have a theory...

[Captain Obvious]

My thoughts on the subject...

First, I would be careful with lithium grease on the master cylinder output shaft. A very little bit right on the very tip would probably be OK, but you don't want any of that stuff to come in contact with the vacuum seal that rides on that shaft. I wouldn't want to assume that rubber seal is compatible with that grease. I think a better choice would be silicone based brake grease. Something safe for rubber seals.

As for sealant on either of the seams between the parts... I don't think sealant is necessary on either of those seams, but a little sealant between the M/C and the backing plate won't hurt anything. And some sealant between the backing plate and the front side of the M/C clamshell won't hurt anything either as long as you don't block the vent passageway.

My read on that vent passage is that it does two things: First, it allows for the change in volume in that (otherwise sealed) cavity when the brakes are depressed. If not for that vent, when the volume inside that cavity were to change, the pressure would change as well. That vent keeps everything at atmospheric. Second, that vent acts as a drain for brake fluid out the hole in the event of a rear M/C seal failure. It would be best if it drains before that fluid ends up inside the master cylinder, but clearly from the amount of fluid inside my failed booster, that's not always the case.

One last thought... If that vent is to act as a drain, it would be pointed down, not up. I don't know if it's the original position or not, but mine was pointed at the 5:00 position, not the 11:00 position. Do you think yours was stock original position at 11:00?