Bleed nipples bad?

So I’m in the middle of bleeding rear inboard brakes on my 88 XJS. The air bubbles won’t stop coming! I’ve gone through 32 oz of brake fluid so far. I was having some leaks in my master cylinder recently, so i thought maybe a lot of air got in the system? But this seems like air is getting in elsewhere. I use a motive power bleeder, btw, at 10-15 psi. No air on source side.

I’ve bled brakes twice before on this car without this much air. The bleed tube is nice and tight on the nipple. It is happening on both sides. But under pressure with bleed nipples closed, there are no leaks anywhere and the system holds pressure.

All I can think of are two bad bleed nipples?

Try a smear of grease on the threads of the nipples, you might find the air is entering there rather than being in the system.

Thanks, I will try that. I’ve also just read somewhere to put the end of bleed tube above nipple so it won’t create siphon and draw air in through threads?

Never had to do that, I always feed into a bottle on the ground.

I’ll try the grease tomorrow. I did notice I could wiggle bleed nipple a bit at the threads as it was bleeding. Never seen this elsewhere.

I dunno how air getting in the threads at the nipple could be a problem. The fluid should be coming out under some pressure, right? So a loose nipple should leak fluid out, not air in.

Doesn’t make sense to me either, but perhaps a siphon from the bleed tube is able to suck air in easier through the threads than the brake pressure is to push fluid out through the threads? I’m only setting the power bleeder to 10-15psi. Should I go higher pressure? What is pressure when one pushes brake pedal?

I’ve never used a power bleeder. Pressure in a brake circuit is certainly much higher than that, but pressure in a reservoir is not. Did you use the pressure bleeder before when you had no problems? At this point I’d be tempted to ditch the pressure bleeder and go back to the tried-and-true.

You’ve passed 1l through, that’s about how much I do when I’m flushing a system which is already bled, filling a rebuilt system I pass about twice as much through, and do it all in one go.

You said ‘no air on the supply side’ are you attempting to pull the fluid out or push it out? If you are pulling it out that is where there can be air drawn into the threads of the nipple.

When I use the power bleeder I always get air past the threads on the nipple, causing bubbles in the suction line. Try the grease trick.

there are 2 kings of bleeders :
suction using depression to “pull” the fluid : those can attract some air in the line, but not in the system : as said above, you need the nipple to be airtight
“push” , feeding fluid in the tank with a limited pressure to get the fluid moving to the bleed points. (like the Eazy Bleed) with those, any bubble out is from the fluid itself, but you need to install them properly and keep the pressure low, or you can spray the whole area with brake fluid

I presume that @gregma is using a pressure bleeder. If it’s a vacuum, then it’s probably air getting pulled past the threads.

Cheap and easy pressure bleed.

1 Like

My not quite .02!

  1. Old tech bleed. Helper pumps pedal and holds. Mechanic cracks the nipple and closes quickly. Neat ones use a hose to a jar from the nipple.> many, just let it go to ground. I always did it that way. Repeat til no bubbles come through.

  2. Older tech. I’ve never used and don’t quite get it.
    fill the reservoir, crack all nipples, walk away for a period. Close and check pedal??

  3. The pressure system to me means apply air at the master. Keep the revoir full. Crack each nipple. NOW, here is my Idea. A leak of bubbles at the nipple thread is just fine. Air is being bled…No suction to bring in air, to the contrary, pressure to push it out!

  4. Opposite to 3. A Mitty Vac at each nipple, one at a time, of course. Vacum sucks fluid and air out. In theory, a leak at the threads is possible. Air is sucked in if the threads are loose!!

If in doubt, replace the nipple!! Nipple metal is softer than the iron of the caliper/cylinder. .Wear from use more likely there/



Thanks Robin! A smear of grease on bleeder threads, bubbles gone! I guess I got an extra liter through the rear brake system, prob good!

I am using Motive bleeder that creates pressure at the reservoir side.

N ow, we will never know. did the grease solve the issue? Or was it a coincidence and the added bleed did the trick.

I have a similar conundrum. Car had start issues. Very unusual for it. determined “no spark”. Cchecked coil wire. Poor. Erred. swapped in spare coil module and a new coil wire. fired instantly, the way it has for a decade or more.

Tossed the bad coil wire. Easy. Was/is the original coil/module bad? could find out by putting it back in?
Will I? Not a chance. Toss??? No back to the spares bins. but labeled!!! Might be bad???

Carl i

I am quite sure grease did the trick. My super experienced friend just told me microscopic scratches develop on the threads. When power bleeding, you’re creating a continuous vacuum. Air can get through the threads whereas brake fluid cannot. He told me he always puts a bit of wheel bearing grease on the bleeder threads to stop this, and keep moisture out.

Tis only academic as the issue is resolved. the air is gone!!!

However, if it is a pressure bleed, where does vacuum come in to it?

If a vacuum bleed, get it. leaky threads…Air enters.,


The pressure creates a vacuum in the bleeding tube, probably narrower passage, or ??? But yes, academic now.
I ended up putting 3 liters of Dot4 synthetic thru the brake system! Probably good for it, as it was sitting for seven years. Brakes feel great, except sheesh this is a heavy car!