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ADW
02-18-2010, 04:37 PM
I recently replaced a rotor and rebuilt a brake caliper because one the brake cylinders was stuck and was gouging into the rotor. I carefully re-assembled everything using brake grease and brake fluid (DOT3) as lubricants were necessary. During the bleeding process a small number of bubbles float up into the servo end of the reservoir. There are no visible leaks in the braking system (from pedal to caliper). The back brakes work fine. I can't get the front brakes bled properly. Is the problem in the master cylinder? Maybe some corrosion? If so, where is the fluid going that is being replaced with air? I see no leaks anywhere. The servo has less than 2k miles on it.

pjsmetana
02-18-2010, 05:18 PM
...I can't get the front brakes bled properly...

What do you mean? The bleed valve is stuck or something? If you can't bleed it properly, then thats where your bubbles are coming from for sure.

ADW
02-18-2010, 07:31 PM
Thanks for responding.


The bleed valves work fine. I can't bleed the brakes properly because air appears to be getting into the system. The symptom is bubbles floating up into the brake reservoir. I'm trying to determine where the air is coming from. The problem is I can't see where the air is getting in. If air is getting into the system, doesn't brake fluid have to be leaking somewhere? I can't see where there is any leak. Make sense?

TOC
02-18-2010, 08:20 PM
Is your master cyliner the one that angles up?
I can tell you from experience, if the master is bad, and not level like other less enjoyable makes, they often don't leak as much fluid out as air in.

My LBC is almost vertical, and when the M/C goes, you don't lose fluid, you lose brakes, and bleed air out for days.
As soon as you quit pumping, air re-enters, and they're gone again.Theoretically, a bad caliper seal can do that, or a bad hose, or rotted line, or, or, or.

Look at the back of the master, where the rod goes in.
If it's wet, it's going to need attention.

SpannerMan
02-18-2010, 08:41 PM
Maybe you need to bleed the master cylinder. Put some pressure on the pedal and crack the line for the front brakes where it comes out of the master while you're holding a rag under there to catch any fluid. (OK you kind of need two people- to tell you the truth I think I did it alone though.) Make sure the M/c doesn't go dry of course. If you can't get fluid to pump out the wheel bleeder when you pump the pedal this may be your problem. The first couple of strokes you may find you're burping out air at the master; then you should get fluid pumping. Of course if you have a nice paint job under the hood you'll want to be very careful about spilling/dribbling brake fluid.

ADW
02-18-2010, 11:35 PM
Very helpful. Thanks. The MC does angle back at 30 degrees above horizontal so your description of the problem fits. I'll check the MC opening as you suggest. It is a little suspicious that this problem appeared after replacing caliper piston seals however I do remember some minor corrosion in the MC when I rebuilt it. I didn't think at that time the problem was severe enough to replace the MC. Maybe now there is more travel in the MC since I now have 4 working cylinders, compared with 3 and 1 stuck in place, the corrosion is coming into play with the MC piston seals.

TexasKnucklehead
02-18-2010, 11:49 PM
Here's another idea. You say you just replaced/bled the lines. I assume you used new fluid. Perhaps there are tiny bubbles in the fluid that you added, and the air is 'collecting' rather than 'entering'. I know if you shake up the fluid, you can see lots of tiny bubbles coming to the top for a long time. Perhaps you just need to keep bleeding it until all the air is gone.

In my TR6, fluid was leaking through the MC and soaking into the floor mat. It took a lot of fluid before I figured out where it was going. Look under the dash for any signs -which are obvious when brake fluid leaks on a painted surface.

ADW
02-19-2010, 10:41 PM
This is a great forum - very helpful. TOC nailed it. The MC was leaking in air and out a (disproportionately)small amount of liquid into the servo/MC joint. There is no sign of leakage at the rebuilt caliper end.

DrEntropy
02-20-2010, 10:36 AM
Any corrosion or pitting in the bore of a hydraulic cylinder is bad. It may seal for a while but ~will~ abrade the seal in time. A brake cylinder hone run thru the bore to approximate the 45* cross-hatch of an engine's cylinder hone pattern is mandatory with any rebuild attempt. If there's any pitting the thing needs either sleeving or replacing.

SkinnedKnuckles
02-21-2010, 04:13 PM
May I modify that a bit? The MC on my car leaked due to a pit at the outer seal. The pit was at the bottom of the bore at the seal "rest" area. The pit finally got big enough to allow fluid to leak around the seal.

Air/moisture gets to the wall of the bore every time the pedal is depressed. Any crud, abraded seal stuff, ect will collect on the bottom of the bore next to the outer seal because the MC tilts back. This is what causes corrosion in that area.

I'm not sure about DOT 4, but if you use DOT 5 that pit can be filled with metal epoxy (I used Devcon) and honed back down to a smooth surface to stop the leak. Rebuilding in that manner is not a safety issue because the seal moves past the repaired area when the brake pedal is depressed before the high hydraulic pressures are produced. If there is pitting elsewhere than at the outer seal rest area, than an new/resleeved MC is required.

DrEntropy
02-21-2010, 07:26 PM
if you use DOT 5 that pit can be filled with metal epoxy (I used Devcon) and honed back down to a smooth surface to stop the leak.

Nice save, Sir. I really-really like Devcon products, have used 'em to do some crazy fixes. Including making heater valve diaphragms with a fiberglass mold when the valves were coming from some ma/pa cottage industry third-world place. It wouldn't be prudent to use it internally on a brake master but the outer seal area would be a perfect place for a fix.

SkinnedKnuckles
02-21-2010, 08:39 PM
Thx. Just offering my $0.05 (inflation).