For those who have changed to a Fiat Master cylinder. How does the pedal feel?

I built my car with one and have never been happy with the pedal feel.

The car brakes well but there is more travel than expected and the pedal never gets truly hard.

After repeated bleeding I let the pads bed in to see if it improved. It passed IVA ok.

I suspect there is still air in there, though I have been to great lengths to get rid of it, or the MC is faulty?

How does yours feel?

In case it matters I have wilwood calipers all round.

Cheers

Are you using DOT4 fluid ?

Yes I think so. Why?

I could be wrong but i'm sure lots of people on here have said that the wilwood calipers are a pain in the ass to bleed

Before doing anything the first thing to do is check all the the pistons callipers and pads are moving freely and that the pedal box master cylinder mounting isn't flexing excessively. If you get a mate/wife to press the brake pedal repeatedly on hard and off you should be able to use your hand detect excessive movement in the pedal box. It is not unknown for brake unions to let air into the system but not leak fluid, very carefully check every union in the system for leakage pressure, using a torch to look for even a tiny amount of dampness.

However I would suspect the callipers are the source of your problems.


Brake callipers have to be mounted so as the pad is presented absolutely (to use the traditional engineering fitters term)" square" to the disc, that is the centre line of the calliper bores has be a right angles to the plane of the disc surface even a tiny error will result in long pedal travel with distinctly springy feel to the pedal. This is because the the calliper mounting is bending to accommodate the misalignment every time the brake is applied and springing back when the pedal is released.
A "quick an dirty" test is get the aforementioned mate to thump the brake pedal on and off while you go round each calliper in turn feeling for tiny amount of flexing of the calliper relative to the disc.
The proper way to check for this is fit brand new set of pads pump the brakes a couple of times, release the pedal pressure and probe round the edges of the pad with a feeler gauge to check the gap between pad and disc . If the gap is wedge shaped you have founf the problem and the only way to fix this is careful hand fettling of the calliper mountings.
Bedding in the pads will reduce the movement but if the calliper is misaligned even after bedding there will always be a torque putting bending load on the calliper mountings and disc.



The other problem is with the callipers themselves Wilwood callipers are notoriously difficult to bleed and as with most alloy multi-pot callipers they flex much more than cast single pot steel callipers. Unbolting the calliper and bleeding the at an angle so that the bleed nipple is at the highest point should help.
Although a dial test gauge is the best tool for the job you could check for calliper flexing with a vernier calliper measuring the girth of the calliper with the brake pressure on hard and then when released

If you want a 100% "for sure"test on the master cylinder you can screw bleed nipples in place of the outlet pipes, give a quick bleed and test if the pedal is hard, unless the pedal box or brake pedal is flexing then the brake pedal should be really rock solid.

[Edited on 4/12/13 by britishtrident]

Thanks for the advice

When I was building the car I spent ages bleeding the brakes and had the calipers at all angles, suspended etc and was certain I had all the air out.

The flexing is certainly worth investigating.

Suggest reverse bleeding the Calipers ..using a cheap disposable 2oz Plastic Syringe .. through their bleeder nipples.
It works ! ...period.
Albeit slooowly... so have patience

http://vimeo.com/29410059

There appears to be some flex in Wilwood calipers!

the flex wont cause too much travel in the pedal. try a remote fluid reservoir that is mounted higher than the callipers then bleed again.

Something else an old engineer i know told me which might help.

He always bled the furthest caliper from the m/c first, i'm sure if this is tosh someone will be along to advise

With a front to rear split just bleed one circuit at a time, then after bleeding both circuits go back round a 4 corners again giving them a couple of pumps.
The very important points of bleeding are (1) just open the nipples just enough to remove excessive restriction to flow, (2) Pump the pedal slowly especially on the return stroke (3) Check & top up the fluid level every 5 stroked (4) Assistant must hold the pedal down to floor while you close the nipples. (5) After bleeding dry and check the nipple for leakage when under pressure.

A pressure bleeder helps but even with a pressure bleeder the pedal needs a couple of pumps to get the air out the master cylinder.

quote:

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Originally posted by ashg
the flex wont cause too much travel in the pedal. try a remote fluid reservoir that is mounted higher than the callipers then bleed again.

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I'm as certain as I can be that BT is correct and it's flex/ misalignment of the caliper brackets.

I remember thinking when I fitted them that even though they weren't perfectly aligned that it would be ok and they would sort themselves out. It would appear not.

I hope over the Christmas break I will get time to have a closer look and do something about it.

I will take off the brackets and make sure the mating faces are clean and free from paint. After that if they are still out they will need to be shimmed. What material would be best to use? If we are talking fractions of mm would gasket paper be ok?

I thought I would update this to bring it to a close.

All the calipers were aligned properly and there was no flex in them either.

I changed the Fiat master cylinder to another new one and the pedal was exactly the same.

Now I should say the brakes were great but the pedal a little long and soft.

I now did more research and reading regarding the size of the MC and using 4 pots all around. This lead me to conclude the master cylinder bore was just too small. Something I think BT pointed out on another thread, that with Sierra rears the Fiat MC was fine but with Wilwood rears not so much.

The Fiat MC is 3/4 or 19.05mm so I looked for a slightly bigger one. Caterham do an AP upgraded 13/16 or 20.64mm MC to use with their AP big brake upgrade but at £185 not exactly locost! A bit of poking about lead me to a Nissan Stanza MC with integral reservoir and float switch in the required 13/16" new for £10.56 inc postage off ebay. It is even made by AP Lockhead!

This MC only has 2 ports one front one rear so a bit of plumbing today and the brakes are bled and FANTASTIC!

Much firmer pedal but still some give so not rock hard like the Sierra MC but still hard enough to make heel and toe much easier.

At last I have brakes I am happy with!

ebay link if anyone is interested?

http://www.ebay.co.uk/itm/BRAKE-MASTER-CYLINDER-TO-FIT-NISSAN-STANZA-/380820050416?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item58aaa4d1f0

There is also Fiat master cylinder available with 20,64 mm diameter. It is TRW-PMF182, Fiat part number 71738443, ATE part number 24.2120-0801.3, Delphi part number LM70135. Also cheap as chips.

That would have made life easier

Have same problem with my mid-engined Alfa 156......changed MC with the bigger one available from Fiat -Alfa - 25mm piston size MC from Alfa 166 V6turbo.......