View Full Version : Brake bleeding problems

05-09-2006, 03:12 PM
Howdy all. In the latest saga of me vs. my brakes, I replaced my servo and master cylinder this weekend. I also rebuilt the rear cylinders and replaced the shoes. The front brakes bled out fine using Speed Bleed valves. The rear brakes, however, have been a nightmare. I've run at least three full containers of brake fluid through each of the rear brakes, and I keep getting air bubbles. It will be fine for a while, and I'll feel peddle pressure build, and then all of a sudden I'll get a lot bubbles. I let the car sit for a day, then tried again, same result, and only at the rear. I don't see any fluid leaks anywhere, so I'm not sure where air could be getting in. I suspect it's either the rear cylinders (which were heavily corroded). Could it be the new master cylinder? I have a new spare (forgot I ordered one, then ordered again, and both arrived within a day of each other).

Any and all advice would be helpful. The frustrating thing is that it's a *lot* of air that comes through all of a sudden. I've also been *very* careful to keep the portion of the reservoir that feeds the rear brakes (and front, for that matter) full.


05-09-2006, 04:07 PM

You could prove me wrong, but I have never seen a case where air is coming in the system, but brake fluid can't leak out. So since you don't see any fluid loss, I think you are still fighting the original air in the system when you pulled the m/c and the rear cylinders.

Did you happen to bleed the front first instead of the back? That could be the problem - work the farthest cylinder out, then rebleed the front and those air pockets will hopefully clear out.


05-09-2006, 05:40 PM
Hi Randy:

Thanks for your feedback. I did, alas, bleed the fronts first. I haven't seen any fluid, but I'm concerned that the new MC is leaking into the servo -- sooo, first, new rear wheel cylinders (which arrived today), then I'll see if that does it. I'm starting to think that it's the rear wheel cylinders, which were scored pretty badly. I honed them out, but I wonder if it was enough. The way the air comes out also raises with me that it's the wheel cylinders. I get bubble-free fluid, and pressure in the pedal goes up, then, all of a sudden pressure will drop at the pedal and a whole bunch of air (mostly air, like milk bubbles frothed when my kids blow air into their cups cup with a straw)will be simultaneously pushed out from the bleeder, as if it had just entered the system close by and were being pushed out the bleeder.

I'll keep everyone posted!

05-09-2006, 06:20 PM
Did you bleed the MC?

05-09-2006, 08:05 PM
I didn't! This is the first time I've installed a new MC in a very very long time (more than 15 years). Sounds like I missed a step! Do tell!

05-10-2006, 07:40 AM
Wish I had read this before I did my Spitfire MC years ago. It took days!


Good luck! /ubbthreads/images/graemlins/thumbsup.gif

05-10-2006, 04:23 PM
Hi Bill,

I think you might be onto something with the slave cyl. at the rear. In general, heavy honing will make the seals too loose to function and air can get past them on when the brakes are released. It's better to go with new slave cyl., if there is any doubt of the fit & finish of the old ones, if anything more than just a little honing is needed. (Note: Sleeving is an alternative for hard to find slave cyl., but most TR SCs are easy to get and cheaper to just buy new.)

If the seals are loose on a slave cyl., or just barely sealing some scoring in the bore, brake fluid might only ooze by slowly under pressure, but air will be a lot more easily sucked in when the pressure is released.

Another thing, are you by any chance using DOT5 brake fluid? It's notorioiusly harder to bleed than DOT3/4. Air bubbles that get into DOT5 stay in suspension for a long time and will gradually combine and work their way out, but that can take days if there are a lot of bubbles introduced to the system during filling.

One trick that helps a lot with DOT5 is to hold a screwdriver vertical in the master cyl. reservoir and pour the fluid gently down the screwdriver blade. That way, far fewer air bubbles are introduced.

The previous response is 100% correct, you need to start bleeding with the slave cyl. furthest from the master cyl., in terms of the length of piping and hoses combined. IRS cars might be a little different from my TR4 (where the left rear is the first to get bled, then the right rear, then the left front, and finally the right front).

Look at an illustration of your brake system in one of the catalogues and you should be able to sort out where to start and what order to work in. Just start with the furthest away from the MC, then the next furthest, etc.

Frankly, I've never bothered to bench bleed my TR's master cyl., have found it easy to get air out of it after all the slave cyl. are done. However, it *might* be different with a servo and the later style MCs, so I can't advise much there.

Finally, I find "Speed Bleeders" to be a real life saver. These replace your bleed screws entirely and have a one-way valve in them to allow one-person bleeding. They are widely available to fit TRs (3/8x24 thread, if I recall, except for the late TR6). Just take your new SCs to any good parts store and you should be able to get them. Slightly longer are easier to work with, rather than shorter ones. I have Speed Bleeders fitted front and rear and on the clutch slave. $25 well invested because they make the bleeding process much, much easier.

Let us know how it goes!


05-15-2006, 10:03 AM
Howdy all! I finally got it sorted out. I thought through the issue and the way air would suddenly seem to be pushed out through the bleed nipple in a huge rush, and decided to replace the rear wheel cylinders. I got them in place, and bled the brakes (rear first) using my Speed Bleeders, and it worked like a charm. I ended up running about a quart through the rear lines, and made sure I only pressed the pedal three or four times before adding more fluid -- that rear resevoir is so darned small! So, either the rear cylinders were the problem, or I finally got air out of the MC. Brakes work fine, though I'll go back and re-bleed them again this weekend to be extra sure.

Thanks for all your insight guys!

05-15-2006, 11:27 AM
Good to hear you got it sorted out; one question though, doesn't anyone use a Mityvac, makes 1 person bleeding very easy. Attached to cyl you are bleeding & pump chamber to create a vaccum, then crack bleeder & let the fluid be sucked out. Works great! /ubbthreads/images/graemlins/smile.gif

I have the basic small unit.


05-15-2006, 12:58 PM
Good to hear you got it sorted out; one question though, doesn't anyone use a Mityvac, makes 1 person bleeding very easy. Attached to cyl you are bleeding & pump chamber to create a vaccum, then crack bleeder & let the fluid be sucked out. Works great! /ubbthreads/images/graemlins/smile.gif

[/ QUOTE ]

I have tried it but all it seems to do is suck in air past the bleeder valve. Tried the pressure bleeder we have the modern cars, but the cap is slightly different. So I seem to always end up with the "pump, pump, hold" method.

05-15-2006, 01:39 PM
I've never had luck with the Mityvac. Since switching to Speed Bleeders, I've really had no use for it, with one exception. The reservoir for the rear brakes is so small that I fill it often, and usually spill some fluid into the part of the reservoir for the front brakes. When it gets too full and I'm still working on the rear brakes, I'll use my Mityvac to suck some of the fluid out of the reservoir. Easier than opening up the front brakes. I also used it to drain the reservoir when I replaced by MC and before I removed and rebuilt my PDWA.