brakes feeling weird.

[JohnL]

From the description it's almost definitely an internal master cylinder leak. Fluid (and pressure) is leaking past one of the cup seals in the MC piston, and escaping back into the fluid reseviour. The leak is internal so you don't lose any fluid from the system and there are of course no external signs of leakage.

I'll also bet that the problem may be worse on hot days or after the car has been parked with the MC picking up heat from the engine bay (hotter when parked or driving in slow traffic with little or no airflow through the engine bay). In my experience the problem will be minor or non existant on cool days...

[redmugenjazz]

thanks john, is this a usual thing for a 35k car. I haven't paid much attention to the brake pressure during hotter days. I'll c n get back to you on that one

[55EXX]

sounds spot on. i am going to upgrade my brakes to 262mm ek vtir etc ones and therefore my MC can you upgrade that and not the rest instead of rebuilding of replacing my existing one?

[JohnL]

I've wondered why this particular problem often seems to be much worse in hotter conditions (i.e. with the MC at a higher temp), and last night (while watching the capitalist system rapidly unravelling on the telly), the posssible answer just popped into my mind.

Speculation as follows;

With a youthful MC the cup seals are soft, 'springy' and of a larger diameter than the MC bore (sort of 'squashed' into the bore), and will seal easily. However with a geriatric MC the seals will be hard(er) with little or no 'springiness', and may well have become 'set' to the exact size of the MC bore, and may only just barely be adequately sealing. But, as the MC warms up it's metal body will expand, and the bore will become ever so slightly larger, so the old hard and 'set' cup seals may now just barely be leaking, enough so that they can't adequately hold pressure, and the pedal sinks to the floor...

[JohnL]

sounds spot on. i am going to upgrade my brakes to 262mm ek vtir etc ones and therefore my MC can you upgrade that and not the rest instead of rebuilding of replacing my existing one?

I don't really understand your question, but I've heard that rebuilding Honda MCs is often unsuccessful (don't know why, I've rebuilt other MCs with no problems). If this is correct then replacing the MC would be the best option.

If doing a brake swap that involves fitting calipers with pistons of different than stock diameter, then if you retain the stock MC you will change both the pedal stroke and the effort required at the brake pedal. If the new caliper pistons are smaller, then the pedal stroke will reduce, but the pedal effort will increase. If the new pistons are bigger then the pedal effort will reduce, but the pedal stroke will increase.

If you change say only the front calipers (to ones with a different piston diameter), then you will change the brake bias which will cause premature lock up of either the front or rear brakes (depending on whether the pistons were increased or decreased in diameter). The same applies if you change only the rear calipers to ones of differing piston diameter.

If you change braking effect at one end then you should also change it at the other in order to maintain braking bias, but even then it might not be so simple. If you increase front brake effectiveness a lot then you may be able to 'stand the car on it's nose' under very hard braking (more so than may have previously been possible), which could unload the rear contact patches enough to lock the rear brakes up unless rear braking effect were reduced (i.e. brake bias moved to the front).

This (reducing the fundamental rear bias) might however mean that rear braking may not be very good in less than very hard braking, so in say wet conditions the brake bias may be too far to the front and so front wheel lock up will be easy to produce (you need more rear bias in the wet due to less forward weight transfer when braking in slippery conditions). This is why cars have pressure limiting valves in the rear brake line, i.e. so that braking bias can be more rearward when not using the brakes hard, but more forward when braking hard.

If you change the front brakes, then you should also change the rear brakes (i.e. from the same donor car as the front brakes), but you should also use the pressure limiting valve from the donor car, because the valve from your car may not suit the new brake set up (i.e. it may limit rear line pressure at the wrong pressure, and so you may get premature brake lock up at one end under hard braking).

[JohnL]

thanks john, is this a usual thing for a 35k car.

Forgot to answer this question.

I wouldn't expect it to happen at such low km, but stranger things happen. Mine started doing it at about 240,000km...

[Benson]

Re-bleed the brakes properly... sounds like there is still air in the system

[redmugenjazz]

Re-bleed the brakes properly... sounds like there is still air in the system

yea i agree i though so, i bleed the brakes in the correct order but i doubt its clear of any air. Do i need to drain the whole system and bench bleed to get rid of all the air????

[JohnL]

Re-bleed the brakes properly... sounds like there is still air in the system

If the problem is caused by an internal MC leak, then bleeding the brakes will make zero difference (unless you also happen to have air in the system). A pedal that sometimes sinks slowly to the floor and sometimes doesn't doesn't sound like air in the system to me. Brakes that need bleeding have a soft / spongy pedal, but it doesn't usually go all the way to the floor unless there is quite a lot of air, and it's usually consistent, not inconsistent as the OPs problem seems to be...

[55EXX]

so i restate my question. down the track i will be swapping my stock eg5 setup for the front knuckle, 262mm disc, caliper, booster, MC etc from a ek4. a common swap into eg's to achieve better braking. i already have rear discs stock and think they would suffice. can you upgrade just the mc to the ek4 one and not the rest (booster, calipers etc) instead of rebuilding or replacing my existing one? i don't wanna replace my stock one to just have to replace it again later.

now from your answer you can but i guess i would have shorter brake pedal travel.

[JohnL]

Yes you can change the MC before anything else and it won't affect brake bias.

If the piston diameter in the new MC is different to the old MC piston diameter, then the pedal effort for X braking effect will change. i.e. smaller MC piston diameter will create higher hydraulic pressures / force applied at pads for X force applied at the pedal pad, and thus a smaller MC piston will result in a lighter pedal that also has a longer travel (pedal will move further and feel spongier).

A larger MC piston will give lower hydraulic pressures / force at pads for X force applied at the pedal pad, so you'd have a heavier pedal that has less travel and feels less spongy. This is because a smaller MC piston applies X force over a smaller area, thus gives higher pressure, and vice versa for a larger MC piston.

In an extreme case (of increased pedal effort with a larger MC piston) you could find that you're not physically strong enough to get full braking effect, but I doubt you'd have this problem. On the other hand, at the opposite extreme end of the scale (with a smaller MC piston) the pedal travel could theoretically increase so much that the MC piston bottoms out before full braking effect is realised, but I doubt this would happen either.

This is all the opposite of what happens at the caliper if the caliper piston diameter is changed, i.e. larger caliper piston gives more force at the pads for X hydraulic pressure / effort at pedal (and longer / spongier pedal travel), and vice versa for smaller caliper piston diameter. This is because a larger piston has a larger area over which hydraulic pressure can act (thus more force at pads), but requires more fluid to be moved to move the pads into contact with the pads.

[55EXX]

after i spread some rep round i'll re rep you. thanks man