I have a rather odd hydraulic brakes problem I hope someone can explain.
I have two pre-war cars. They are both fitted with the brakes off a '30s Triumph Gloria which are normally more than adequate. However, for various reasons they have both had brake rebuilds in the last six months. They now both exhibit the same fault.
The symptoms are that after a week or two the brakes go very soft ? to the extent that hard application of the pedal is capable of running out of travel. Pumping them makes very little difference. However, I can cure the problem by bleeding one slave cylinder ? it is always the same one (but not the same one in each car ? one its offside rear and the other its nearside rear). No air appears to be present in the fluid ejected, and it only requires a couple of pumps for the pedal to be rock solid again, until the next time.
It loses no fluid while sitting, so my question is how can I be getting air into a hydraulic cylinder while it is sitting in the garage? If a seal was leaking there would be a loss of fluid, which there isn?t. Is it possible for a slightly reduced pressure to occur in the system so that a relaxed seal permits a tiny amount of air? I can?t see why, but it?s hard to think of any other explanation.
Does anyone have any suggestions? I intend to replace the seals in the affected cylinders as soon as I can source some ? but I would really like an explanation as to what is going on. Unanswered questions always leave me unsatisfied!!!
|
Classic symptoms of leaking master cylinder seals. It is an internal leak, so no fluid is actually lost.
Usually what happens is that the brakes work well for years, then after some minor work, such as replacing a slave cylinder or a rusty or perished pipe, you bleed the brakes, and then get the symptoms you describe.
What has happened is the the section of the master cylinder piston travel in regular use is kept nice and clean, but the unused portion gradually becomes rusty or accumulatesdirt. Then you push the pedal to the floor in bleeding the brakes, and you run the seals over this unused section. This scores the rubber slightly - not enough to destroy them, just enough so they leak very slightly.
It is a cautionary warning to never, ever, push the brake pedal beyond the range of its normal travel. Pressure bleeding avoids this.
Your problem is slightly different in that it appears that bleeding a particular slave cylinder will alleviate it temporarily. I can only assume that the act of pumping the pedal restores some of the lost sealing ability, but that this gradually subsides with time.
Other possible factors to investigate:
Binding shoes, causing localised heating and hence aeration at one cylinder.
Air stubbornly trapped in an air lock somewhere.
Ballooning rubber pipe, perhaps exaccerbated by localised over-heating.
Pipework getting heated by engine or exhaust. Have you switched to copper pipes perhaps?
Unsuitable fluid/seal combination?
Time-expired original new-stock seals?
Over-bored cylinders?
|
Hm! Thanks for that, lots to think about.
I?d have thought that, if it were master cylinder, then pumping it a few times would restore pressure without bleeding being necessary. Also, bleeding of any brake should work. - it appears to be specifically one slave cylinder that has the problem, and it only takes a couple of pumps to restore pressure. Flexible pipework is all new, stainless shrouded Teflon, so shouldn?t be ballooning. Can?t be temp. dependent, as it happens when parked unused in the garage - which is the bit I find really hard to understand!!!!
I could believe the time expired new seals, I don?t know if they are original or replacement parts, but if so why only air in, and not fluid out (and how!!!). I think maybe first action is replace the seals in the affected slave cyl, and if it still does it then strip the master cylinder.
|
Could it be to do with the return springs on that wheel? I imagine there must be a small residual pressure in the system, which posibly keeps seals expanded and sealing? Or perhaps it is meant to be totally relaxed, and a residual pressure causes slight leakage?
Presumably after a run the system cools, and has to draw in a bit more fluid from the mastercylinder. Could a leaking master cylinder seal, or a weak or incorrect spring in the MC, mean that a slight vacuum develops instead, drawing in air at the weakest of the four sets of cylinder seals?
I recently read a detailed account of the working of the MC in my Triumph 2000, about the role of the internal spring in resetting the MC piston rod to the correct position, so that an internal transfer port was uncovered, under back-pressure from the wheel springs, to allow a fresh top-up from the reservoir. Apparently if the seals are leaking, or the spring tension incorrect, the pedal simply goes up and down but doesn't pump its full charge of fluid because it partly fails to re-charge.
Sorry, lots of possibilities and unknowns, not a lot of definite answers.
|
|
|
|
Are you certain the pipe that goes into the s/cylinders is sealing correctly,I have known them to be slightly oval and although they dont leak fluid,they leak air into system?
Its only a thought but has happened before
--
Steve
|
When bleeding the relevant cylinder, is the fluid clean?
Any sort of cloudiness could mean that seal in that cylinder is deteriorating and allowing air into the system.
|
|
try clamping of all four flexis then pressing the brake if it goes down the master cylinder is faulty
|
|
Replace the wheel cylinder that is causing the fault ... (only clutch hydraulics have a slave cylinder) ...
|
On re-reading pvmw's post, he'd already decided to replace the relevant wheel cylinders but wanted an explanation of why a faulty w/cyl should affect the system in the way that it does.
I've come across a similar fault a couple of times in the air / hydraulic systems I work on. It seems that the faulty w/cyl seal allows air into the system, but that the air gathers close to the bleed nipple area. This means that that when bled, the initial flow of bubbles and air, usually associated with an empty bleed pipe, can be air being expelled from the w/cyl. The real clue is the brake fluid. Cloudy or containing small black bits means seal failure.
|
Further to Cliff Pope's first post here: you have made the point about never pressing the pedal past its normal lowest point to avoid damaging the seals. This has never happened to me, but I certainly take the point that it can happen.
What I want to know is, can one find a pressure bleeding kit that won't cost the earth? It isn't all that complicated as an idea.
|
What I want to know is, can one find a pressure bleeding kit that won't cost the earth? It isn't all that complicated as an idea.
There is one called Gunsons eezibleed. Teens of UKP
Powered from the spare wheel (reduce pressure first to less than 20 PSI).
Can be a challenge to get an adapter for the top of some old master cylinders, although there are a number provided.
|
|
Thanks AJH. What does 'Teens of UKP' mean?
|
Thanks AJH. What does 'Teens of UKP' mean?
a cost between 13 and 19 units of UK currency .
Depends if you buy on line, or Halfords, which end of the price range you pay.
I didn't use the pound sign so that someone with a non UK key map might be able to make sense of the comment.
|
|
