dcwang3

Does anyone know what the torque spec is for the master brake cylinder, where it attaches to the brake booster? I am planning on changing out the gasket as I have a very long whistling noise and don't want to over torque it.

aptoslexus

9 ft/lb should do the trick.

timmy0tool

iTrader: (7)

whistling? there should be no air leaks at the booster since it's a closed system independent from the MC.
you have bigger problems if air is leaking at the booster and changing the gasket is only band aiding the situation.

likely the fluid has escaped the MC and leaked onto the booster, causing it to corrode thus the air leak.

dcwang3

So I did replace the gasket but there is still a leak at the bottom side of the gasket. I’ll take another look at it but I didn’t now see any fluid or any corrosion around that area. I guess another question would be is the rubber boot that goes at the end of the MC suppose to be another seal against air leaks or what is the intention of it, it’s part 47265.

also the brake fluid level hasn’t changed and has been at max, no noticeable change.

im a little skeptical that the small thin paper gasket is suppose to be an effective seal between the MC and booster.

timmy0tool

iTrader: (7)

that paper gasket really isn't meant to be air tight. I suspect your booster seals have failed where the push rod meets the MC.
the brake fluid is independent from the booster which is why the fluid levels are steady.

think of it this way:
booster = air, vacuum from the engine powers it
master cyl = fluid, from the reservoir then is pushed to the brakes

both work together, but air and fluid are not mixed!
the rubber boot at the end of the MC is to keep debris from messing up the seals.

oldskewel

iTrader: (1)

In case you decide to do it, rebuilding the MC is very easy to do. PN for the lexus rebuild kit is 04493-22220 (for the '91 at least), was around $50 when I did it on my '91. When I did mine, it turns out it was not needed, but nice to be done. Sometimes it's easier to just do something cheap and simple than wonder if it's needed and causing a problem.

dcwang3

So then if I look into the booster near the tip of the push rod, there should be a rubber seal. I assume this is built into the booster and can’t be replaced unless the entire unit is replaced.

would it matter as a quick fix to use rev gasket maker around the MC flange to make that section air tight rather change the booster at this point in time. Functionally the booster works fine and no issues with braking.

oldskewel

iTrader: (1)

I'd assume that if it's made of rubber and it's in the MC, that it will be included in the rebuild kit. Pretty sure of that, actually.

The fact that your brakes are working fine is an important factor here. I missed that earlier. If you make sure the RTV does not get anywhere it is not supposed to be, I don't think it would hurt, and might solve it.

Have you tried isolating exactly where the whistling is coming from? If you think it's coming from the bottom of the booster, you could use a thin flexible hose, with one end held up to your ear and the other end held down where you're probing for noise.

dcwang3

so if the Booster is suppose to be sealed at the push rod, then hypothetically does that mean if I take off the MC and turn the engine on, the booster should still be air tight since the MC gasket is technically not doing much.


thanks for letting me know I’ll keep that in mind. At this time I don’t think It’s needed because it’s not leaking and braking feels normal.

dcwang3

the air is being sucked in at the bottom where the MC mates with the booster. That leads me to my other post above asking about taking off the MC and running the engine to see exactly where it’s coming from

timmy0tool

iTrader: (7)

yes you understood correctly; you should be able to run the engine with the MC off and you will be able to see/hear the leak source at the booster.

dcwang3

I actually did that and found that the seal directly around the shaft of the booster was cracked and damaged. Looks like I’ll be putting in a new booster.