Loss of brake pressure, but has fixed itself?

[daverclasper]

Had a loss of pressure, partial and straight down to the floor. Would build pressure back easily with one or two pumps, though lose pressure again moments later.

Thought it likely the small seal at rear of Master cylinder chamber had failed.

Obviously stopped using car. Next day I tried it and seemed fine, went for a couple of slow drives around the block, using the brakes a lot, though stopping at times for a bit as well, to see if they lost pressure.

Appears to be no loss of fluid. Thinking maybe seal hasn't failed, though maybe a bit of crud behind it at the time?.

Seals have been in there about 8 years (reluctant to change, as a matter of course, due to likely repro crap) and changed fluid 3 times in that period, (last time maybe 3 years ago).

So, if it is a bit of crud, maybe clean out Master Cylinder?, or any other advice would be great please.

Thanks, Dave

 

   

[dougbgt6]

Dave,

When did you last change the brake fluid? is it Dot4 or similar? Dot 4 absorbs water and can boil if driven hard, result no brakes, until the steam cools down and back into water. Dot 4 should be changed every 2-3 years. Uncle Pete has a nifty sensor meter, he lines the Beds faithful up and ceremonially tests their master cylinders on BF Test day.

Doug 

[Pete Lewis]

i doubt Dave drives hard enough to vapourise moisture laden fluid  and he has changed the fluid in the past   

this sounds more like a seal failure in the M cyl I think a repair kit rather than chance it does it when you realy need to stop ...and  .bang   

Pete

 

[daverclasper]

On 28/10/2022 at 19:16, Pete Lewis said:

i doubt Dave drives hard enough to vapourise moisture laden fluid 

You know me Pete (though when you overtook me on the M way going to Twiddle Day and doing a disgracefully slow speed of about 50mph, I was a bit lost and trying to look at road atlas while driving, I would have been powering along at 60 otherwise).

Yes, best to change seals.

The thing that made me think there maybe ok (and maybe just debris behind the small seal), was that the pressure was lost while stationary (rather than harder braking) and didn't lose pressure again when I tried the next day, even when braking hard (though not at fast speed). Though I don't enough about this, for example, does that seal have to hold a lot of pressure (out of interest)?

Thanks for input.

[Peter Truman]

On 29/10/2022 at 04:00, dougbgt6 said:

Uncle Pete has a nifty sensor meter, he lines the Beds faithful up and ceremonially tests their master cylinders

Snap we used to do the same here in our Triumph Club!

My local brake specialist used to change the fluid and bleed for $60 not worth getting my hands dirty! Unfortunately he retired & nowadays there's a dearth of brake specialists, the chap who brought the business after Covid only sells parts no service anymore.

[Unkel Kunkel]

We can speculate as to causes - whether  boiling brake fluid, stuck m, cylinder valve , combination of factors etc etc.

but it has to be seen for what it is - a hydraulic system failure,( even if some function was recovered).

To have confidence restored  following such an incident,  an overhaul with seals or cylinder replacements  and a fluid change would be the next step for me , I think.

 

 

[johny]

10 hours ago, daverclasper said:

You know me Pete (though when you overtook me on the M way going to Twiddle Day and doing a disgracefully slow speed of about 50mph, I was a bit lost and trying to look at road atlas while driving, I would have been powering along at 60 otherwise).

Yes, best to change seals.

The thing that made me think there maybe ok (and maybe just debris behind the small seal), was that the pressure was lost while stationary (rather than harder braking) and didn't lose pressure again when I tried the next day, even when braking hard (though not at fast speed). Though I don't enough about this, for example, does that seal have to hold a lot of pressure (out of interest)?

Thanks for input.

Both the main seal and reservoir return seal have to hold back the same full braking pressure. Heres the diagram from the manual and the small seal is part of a fairly delicate assembly which you need to check is still functioning correctly once the new rubber part is installed...

[NonMember]

Note also that the reservoir return seal is held closed mostly by that hydraulic pressure. The only thing closing it before that is the wavy washer, part 3 in the diagram. The operating rod, part 5, serves to open it when the pedal is fully released.

[Pete Lewis]

as for pressures its years since i played with that on trucks with air over hydraulic 3500psi was quite normal  on a car with no assistance 

take the pedal effort at 50lb then the pedal ratio is around 5:1  thats  250 lb at the pushrod    i could gauge up the hydraulics but im not going there 

its sunday  roast beef is about to sizzle 

Pete

[johny]

2 hours ago, NonMember said:

Note also that the reservoir return seal is held closed mostly by that hydraulic pressure. The only thing closing it before that is the wavy washer, part 3 in the diagram.

Yes I think thats why sometimes a slow pedal push can cause the port seal to leak but a stamp and its ok. Luckily thats the normal reaction if this happens driving😂

[NonMember]

1 hour ago, Pete Lewis said:

take the pedal effort at 50lb then the pedal ratio is around 5:1  thats  250 lb at the pushrod    i could gauge up the hydraulics but im not going there 

The cylinder bore is 5/8", so the piston area is Pi*(5/16)^2 = 0.3 square inches. So 250lb gives about 815psi

50lb pedal effort is quite mild, though.

[PeteH]

Replacing the Brake fluid circa every 2 years or so is not a bad idea, on any vehicle. Recomended by most manufacturers. I had a very "brown trouser" 5min on a hill on the old A18 some years back, I only stopped when I got into the village and jambed the tyres in the kerb of a bus stop. Brake fluid Boiled. That was a lesson learned. We had not long been back from Europe and over the Pyrenees in it.!

Pete

[Unkel Kunkel]

Brake failure, fifty years or so  ago, driving my father’s Ford 105e Anglia.Hot weather, very long journey  across Lancs, Yorks and on to  Lincolnshire coast.

Pedal to the floor.. No foot brake!

Old fluid boiled, I think, but what ever the cause, the  master cylinder spewed  its contents out  through the hole in the master cylinder  cap - over much of the under bonnet area.

Nothing dramatic, really.

.. but thankful for a reasonably good handbrake.

 Brake Hydraulics, especially brake fluid = Respect !

 

 

 

 

 

[Peter Truman]

Early 70’s touring Europe in the Vitesse going over the St Goddard Pass from Lucerne on the down side to Como an Alfa Montreal was trying to pass me he caught me between bends but was slower out of the bends when he finally got past I had virtually no brakes also the steering wasn’t right.

When I got to Como there was a top end garage next to the hotel I took it there they very helpfully filled and bled the brakes and fixed a broken steering rack clamp, cost NIX the mgr said he’d add it to the next Roller bill!

Since then I change the fluid every 2 years on all classics except the Vitesse which I use Silicone in.I also have a moisture level tester and check at least twice a year.