Brakes spongy

[MikeyGordon 10]

I just bought a Rado 20v today, it has wildwood callipers up front but the brakes feel very spongy.

I noticed that the master cylinder looks tiny compared to the 23mm ibiza master cylinder I fitted to my 20v mk2 golf.

Do any of you think a simple bleed of the brakes will sort the feel, or would I be better off upgrading the m/c?

[mimjed 0]

Always worth a bleed as it is cheap. You can upgrade master cylinder, May be worth doing servo too. Mk3 tdi's are the same size but you have to cut the shaft on the back of it and weld the old one on (corrado is a clevis pin).

 

Ford focus also fitted with 23.8mm master, they fit too.

[MikeyGordon 10]

Aye I will get them bled before it goes for its MOT, hopefully it makes enough of a difference. Does the Rado having abs affect the use of the focus/ibiza m/c? There is 2 port and 4 port versions available I think, what would be the best one to go for?

Thanks.

[mimjed 0]

If you are keeping abs then you need a two port. Mk3 golf/ibiza or Ford focus with all fit, and have the bigger cylinder. Think the only thing that may change is the union size on the master, up to m12x1 instead of m10x1.

 

They are essentially the same in braking terms, just comes down to price. But on the basis you are 20vt I'd suggest a stubby out of the Ibiza would be best suited due to engine clearance.

[MikeyGordon 10]

Im sure the 2 port uses 2 m10x1's, a lot of people use them for easiness in the conversion but I used the 4 port and made up 4 independent lines instead.

Thanks.

[mimjed 0]

Hmm always thought they went to m12 on the abs models to allow more flow.

 

I have a 4 port stubby from Ibiza on my rado. 2 m10 ports on left hand side, 2 m12 ports on right side. I'm using the m12 for the fronts for the same reason - more flow.

[Sean_Jaymo 0]

The size of the bolts is to stop connecting the wrong pipe to the wrong hole. 1 circuit will be a 10mm and the other will be 12mm. It so that if one of the circuits fail, you don't loose both.

 

The pipe diameters are still the same. It's not flow you want, it's pressure. More flow equals less pressure.

[MikeyGordon 10]

You're probably right mate, I just thought they were m10x1 as a lot of mk2 owners used them to save changing any brake lines. Im using the same set up as you though.

[mimjed 0]

Not sure I follow, holes are bigger in the banjo bolts to allow more fluid to move through the pipe with the same amount of pedal movement. In terms of hydraulics you get more push as a result.

[Sean_Jaymo 0]

The internal diameter of the pipe is still 3/16. A master cylinder won't meter out 2 different internal pipe diameters, it's up to the abs pump or compensating valve to meter out flow to the pistons.

 

On non abs cars, brake pressure is the same left to right irrespective of how much the car is turning which is why it's easier to lock up the inside wheel. That's where the advancement in brake systems to abs allows for wheel specific brake forces.

 

Apart from the Corrado that is. It's only 3 channel, front left, front right and both of the rears are treated as one.

 

More modern cars have 4 channel systems to improve further.

 

More flow equals less pressure so you will need to push the pedal harder for the same amount of pressure to be applied at the caliper. The caliper only accepts a certain value of fluid, enlarging the diameter of the pipe allows a larger surface area of contact within the pipe for the fluid to act upon and so the force dissipates against the inside of the pipe and not on the caliper.

 

Hope I make sense. Fitting a larger master cylinder will not improve braking unless you stamp on the pedal, it will make the pedal travel feel worse if anything. A larger master cylinder reduces pressure. You will have a harder pedal with less pressure at the caliper. Not ideal really.

Edited January 12, 2015 by Sean_Jaymo

[mimjed 0]

whilst some of the points you make are true, i believe you are missing the principal. i don't claim to be good at physics but...

 

if a piston of 5cm pushes down 10cm it displaces as much fluid as a 10cm piston moving 5cm. I'd presume in the world of physics that would probably mean more force would be required on the larger piston to achieve the same result, but as we are talking about going from a 22.2mm to a 23.8 the difference in force wouldn't be noticeable by your right leg.

 

therefore you achieve more breaking force with the same pedal travel. this reduces the spongy feel and makes the pedal more reactive. so no stamping required on the pedal.

 

also the only parts left of a corrado braking system i have is a pedal, handbrake cables and an abs light that will never turn on. everything has been changed to improve on brake pedal feel and performance.

[Hairyarse 10]

That's not correct on the pistons as they're round, not square. The swept volumes are 196 and 392cm3 respectively. The rest of the debate I'm clueless on but reading with interest! lol ;)

[Sean_Jaymo 0]

you achieve more breaking force with the same pedal travel.

 

This is correct but if you were to push as hard (not as far) with a smaller master cylinder you would exert more pressure on the brake caliper.

 

You and I appear to be talking brakes in a slightly different tone to each other. I am describing how much brake force you exert on the caliper for a given pressure and you are talking for a given distance of pedal travel. Personally I would prefer for the brakes to work harder for the effort.

 

Let me try to explain my logic with some maths.

 

Surface area of the 22.2mm piston is 387mm square or 0.387 metres squared

 

Surface area of the 23.8mm piston is 523mm square or 0.523 metres squared

 

This is an increase of about 34%

 

Lets say for arguments sake that the fulcrum point is 1/5th of the way down the brake pedal. It's irrelevant as the fulcrum point is the same for both master cylinders but it would leave a brake pedal ratio of 5 to 1. The basic formula for pressure on the calipers is

 

Pressure = (Force on pedal x pedal ratio) divided by master cylinder piston area in metres square - For arguments sake lets stick force on pedal as 20KG.

 

So that's

 

(20kg x 5) divided by 0.387 or 100/.387 = 258psi

(20kg x 5) divided by 0.523 or 100/.523 = 191psi

 

Again that's a difference of about 34%. This means that for a given force on the pedal you would get a REDUCTION of 34% in pressure by increasing the master cylinder size. As hydraulic fluid is not compressible, I would assume that you would have to press 34% harder to get the same amount of pressure at the caliper which means you would have to put 27KG of force into the pedal to stop as quickly.

 

You are right though, you would get a firmer pedal quicker as the larger piston would displace the fluid quicker but the increase in piston size means that the pressure you are putting on the pedal is dissipated over a larger area and so the stopping effects would be reduced.

 

I'm sorry if I come across as patronising, I'm not trying to be. I'm simply trying to back up my point that larger piston sizes do not mean an increase in braking force, it only reduces the pedal travel but with the trade off being reduced pressure.

 

If you were to increase the size of the vacuum servo, then you would get an increase in brake assistance and so an increase in brake pressure with less travel. This is how most modern cars give the feeling of stronger brakes, larger servo tied in with a larger cylinder.

[seanl82 23]

Well backed up Sean. I agreed with you on the principle, and did have a quick look for my course handout for hydraulics to back it up. I couldn't find it, and I'll be damned if I could remember anything like to this level of detail without researching! Hats off to you Sir! :salute:

[Sean_Jaymo 0]

Cheers mate, aircraft techie by trade and have om15 running through my veins!

[MikeyGordon 10]

All this is waaaaaay too techy for me!

I'm just a daft abseiler haha

So the moral of the story is the master cylinder should be fine?

I will try giving the brakes a good bleeding.

There is braided hoses almost all round so can't be the flex of the flexies ;)

[Sean_Jaymo 0]

There is a bleed nipple on the master cylinder that normally gets over looked when bleeding. Being the highest point in the system, you'll struggle to get it out any other way!