Brake pedal slowly sinks to floor under pressure

[Adrian]

Hi all

Not quite the same topic as recently submitted (how good should our brakes be) but on that note - new pistons in rear, adjusted and new lines to front. Bled and Dot 5 installed. I currently can't get to all the joints to check but reasonably confident that they are pinched up ok. SO when testing resistance at the brake pedal I notice that with continued pressure the pedal slowly sinks which to my mind means that there is fluid leakage around the seals in the Master?

Assuming this is correct ,what would you suggest  - new seals for a rebuild, new Master or .............light  the blue touch paper...............Servo!!!!

I think the servo could be a lot of additional fitting so easier and quicker to keep standard. If I can lock the wheels after i'll be happy with any solution.

Thanks

Adrian

[Pete Lewis]

if no leaks on the renovated then this is either the recouperation or main seal in the master cyl. especially if a stab hold and steady drops 

the recoup seal is the small one that has to close the port to the reservoir , often the weak link 

Pete

[NonMember]

It's most likely the recuperation seal, as Pete says. I had the same on my GT6 clutch (the symptoms are a little different as you expect the pedal to go all the way but sitting on the clutch for any length of time it gradually re-engages by itself). The only fix is a new seal - whether by refurbishing your existing master or replacing with new. Adding a servo will make no difference to this at all.

[dougbgt6]

Adrian,

Refurb kits for the master cylinder are cheap enough and worth a try. £7 as opposed to a new MC, £80! Make sure you get the right kit.

And, speaking as an arch servo enthusiast, I agree with NM, it will make no difference to this problem, get it fixed, THEN get a servo!

Doug

[daverclasper]

9 hours ago, Pete Lewis said:

the recoup seal is the small one that has to close the port to the reservoir , often the weak link 

Hi Pete. When I first got car, I replaced the clutch and brake M/C seals, as a matter of course. Both seemed to have very good bores (with a bit of of polishing to add to it).

Soonish after, the clutch lost total pressure on a couple of occasions (2nd time after 2nd seal kit) and I replaced with £10 landy cylinder (which also gave a bit more clutch, as was crunching into reverse on occasion before).

Replaced the brake M/C seals a bit back, as leaking slowly from large piston seal (got a spare kit, in case it happens again on holiday, though these mains seals it seems are usually slower burners and you can get away with topping up the fluid for a bit)?.

So, after all that waffling, here's the main question eventually (thanks for keeping with this , if you have) , is a M/C complete failure, normally down to the recoup seal (or maybe something else stopping the  recoup sealing properly)???????.

Cheers, Dave

   

[Pete Lewis]

you can get alignment problems with the recoup seal if the thin wavy washer it sits on gets trapped out of place 

apart from that its simple and a fail is a fail one of the two seals 

Pete

 

[NonMember]

If the recup seal fails badly then you get total failure but without loss of fluid. More often, they fail a bit, and you get weak effort and/or slowly receding pedal like Adrian described.

[johny]

You might be able to confirm a recoup seal leaking by checking if the level in the reservoir goes up slightly as the pedal sinks to the floor. You will have to mark the level somehow before an assistant presses the pedal....

[Tom]

My Herald did exactly the same a few weeks ago, no leaks anywhere, no fluid loss and slowly sinking pedal.

My car had stood for 8 odd years, I think I changed the fluid once in that time to save the brake system but still ended up replacing the rear cylinders, front pistons and seals.

I stripped the MC, bore was perfect, seal not split but had become a bit squidgy and swollen, cleaned it all up popped a new seal kit in, had a good few hours of bleeding ( that was the hardest part of the fix! ) but now the brakes are the best they have ever been!

[chrishawley]

Beware DOT5, it's not necessarily any advantage over D0T4.

DOT4 is quite hydroscopic whereas D5 barely absorbs water at all. On an aged brake system there may be minute amounts of residual water and it is a Good Thing to have this absorbed into the fluid. Otherwise one may get a water bubble which will then varporise under pressure and become compressible vapour. So on an older system D5 may make matters worse rather than better.

 

[dougbgt6]

Chris,

Dot 4 should be changed every 2 years BECAUSE it absorbs water. Absorbing water is not a “good thing” it will still boil just as easily.

Doug

[Clive]

Here we go again......

[Adrian]

I suppose you could look at it as the DOT4 has already absorbed the water and consequently there wouldn't be any left behind during a change to DOT5. If there was any water left behind then it would be no better or worse than if DOT4 remained.

I was dubiouse at first about the change particularly if you broke down but most breakdown services would either carry the correct stuff or relay you home anyway. But the positives are the fact that my cars are not daily drivers and I don't have to worry about the fluid degrading AND any leaks wont strip the paint!

I understand that schisms exist on multiple topics and it is importantly down to individual experience. I'm just glad that we can discusse ALL opinions / options openly so that we can make informed 'eyes wide open' desicions and then come back to the forum when it all goes wrong!

Cheers

Adrian  

[ahebron]

 

1 hour ago, clive said:

Here we go again......

🙈🙉🙊

[johny]

I thought the difference was hydraulic fluid absorbs any water and so its more dispersed through the system while silicone being non hydroscopic and lighter leaves it to collect at the lowest point. For this reason it's difficult to remove the water by bleeding a silicone filled system and the water can remain to cause corrosion damage...

[dougbgt6]

This is about the best review of brake fluids I've read. Buckeye Triumphs

https://static1.squarespace.com/static/5c6dec53b10f25d4edf0b3f7/t/5d01933a9c74c30001818fe4/1560384315450/Selecting+Brake+Fluid.pdf

Doug

[Pete Lewis]

and also 

https://epicbleedsolutions.com/blogs/faq/whats-the-difference-between-dot-4-and-dot-5-1-brake-fluid

pete

[Clive]

4 hours ago, Adrian said:

I suppose you could look at it as the DOT4 has already absorbed the water and consequently there wouldn't be any left behind during a change to DOT5. If there was any water left behind then it would be no better or worse than if DOT4 remained.

I was dubiouse at first about the change particularly if you broke down but most breakdown services would either carry the correct stuff or relay you home anyway. But the positives are the fact that my cars are not daily drivers and I don't have to worry about the fluid degrading AND any leaks wont strip the paint!

I understand that schisms exist on multiple topics and it is importantly down to individual experience. I'm just glad that we can discusse ALL opinions / options openly so that we can make informed 'eyes wide open' desicions and then come back to the forum when it all goes wrong!

Cheers

Adrian  

The thing that concerns me g=here is that it sounds like you just changed the fluid, but not all the seals in the hydraulic system. 

And I very very much doubt any breakdown service carries dot 5. It isn't easy stuff to buy off the shelf.

As to water droplets, the bore of the brake pipes is very small so they should be bled out OK. Not so sure when it gets to a caliper though as the bleed is always at the highest point. And the place the water is most likely to get boiled. But ver few people drive their cars hard enough to do that, those that do will almost certainly use normal brake fluid. So a non-issue I reckon.

[Pete Lewis]

how many modern   manufacturers  deviate  from Dot4  ???   its good at abs response times etc  

back in the days of lockheed and girling anything  examined with any other than D4 and warranty disappeared 

Pete

[Adrian]

I have an old Land Cruiser as my everyday driver and it must be 6-8 years since any brake fluid change. I just ask for a boil test. My brakes are still as sharp as anything. Maybe the modern systems have better seals!

PS I've just received a seal kit from MevSpares.

[mark powell]

PS I've just received a seal kit from MevSpares.

 

 Is it well wrapped in brown parcel tape?   (all my orders from them are...) 

[Pete Lewis]

water content  boiling /vapourising of Dot4 Dot5.1 is nothing to do with any seals its all down to very hot brake pads passing the heat into the caliper and  boiling the moisture content absorbed in the fluid 

replace every 2 years is the standard use by the industry ( old or modern ) , if you never get your brakes really hot its wont happen to you , doesnt make it right 

i check members cars with  a cheap meter and most at 2 yrs are heading for 1.5%            2.5% is considered dangerous

this is reservoir content the main system could be more or less but its all absorbed from normal atmospheric breathing via the cap vent 

dont ask what happens with silicon  it wont absorb moisture but any  gained from vent breathing is still a problem

we had to check trucks which tend to have long storeage and it was easy to find 20% , air temperature changes cause breathers to take in moisture ,  so  classic cars standing for long 

periods of lay up /less use do not escape 

its a cheap fix  relatively easy  DIY  to do  dont be fooled into thinking, Its Not mine  !!!!

Pete

[Adrian]

5 hours ago, Pete Lewis said:

boiling

The garage told me the boil test was to measure the dissolved water content (boil small sample and determine volume loss, lost volume=water) most likely with a view to determining the 2.5% danger level. 

[Pete Lewis]

from £5 to £14  tells you in a split second 

get one for Christmas   

Pete

[daverclasper]

As I'm not much of a racer, or hard, continuous braking,

I have been changing it every 2/3 years. For me it was about it not encouraging internal corrosion in my steel pipes and cylinders?.

The old fluid is usually a bit less clear than fresh stuff.

I know it's brakes and I'm as much concerned about possible failing due to corrosion, as boiling.

I bought the pen type one off Ebay a few weeks ago for about £2, inc delivery. 

Cheers, Dave 

 

Edited June 27, 2020 by daverclasper