Hi-Spec 4 Pot Calipers, M/c To Use For Them

[Pugmanian Devil 1]

Hi guys,

 

I am having a bit of a problem with my brakes and have been told mixed information about what size master cylinder to use,

the story goes...

 

 

The front - 285mm discs, Hi Spec billet 4 radial calipers 4x34.6mm pistons per caliper, ferodo ds2500 pads

 

The rear - standard rear calipers, disc's and pads (being fed through a AP racing bias valve)

 

Current master cylinder 19mm 4 port

 

 

Now the problem i am having is the first inch or slightly more of the brake pedal does nothing then after that the brakes progressively start been applied with good feel up to a certain point then the pedal starts to feel hard'ish, at which point the brake performance feels weak (like the car does not want to stop)

 

Can somebody suggest what the correct size master cylinder should be used for the size of the caliper pistons and disc's etc, Hi spec say a 19mm, other people have said a 23.8mm others 22.2mm

 

Can the master cylinder affect brake performance like what i am experiencing?

[camgti 56]

I had similar problems to what your experiencing when I went from standard brakes to 266mm S16 brakes. People told me 19mm or the 22mm would be fine. I bought a new 22mm fitted that, not much difference.

 

Decided to try the 496 non abs 23.8 and they are now amazing. Solid pedal from almost the top of the pedal.

 

It was important to use the lager 306 rear calipers to match the braking fkrce front to back for my setup. But as your running a bias valve you should be fine.

 

Cam

[Pugmanian Devil 1]

I had similar problems to what your experiencing when I went from standard brakes to 266mm S16 brakes. People told me 19mm or the 22mm would be fine. I bought a new 22mm fitted that, not much difference.

 

Decided to try the 496 non abs 23.8 and they are now amazing. Solid pedal from almost the top of the pedal.

 

It was important to use the lager 306 rear calipers to match the braking fkrce front to back for my setup. But as your running a bias valve you should be fine.

 

Cam

Glad to hear you got it sorted, i was leaning towards the 23.8 m/c. do you happen to have a part number for the one you bought? and did you mean "406" ??

 

Thanks for the reply

[camgti 56]

No part number on hand. Do a search and youll find it.

 

Yes, 406 non abs MC.

[camgti 56]

Found this quick

 

http://forum.205gtidrivers.com/index.php?showtopic=56853

[Pugmanian Devil 1]

I got one today and fitted it, i am yet to start the car and drive it but the brake pedal does feel that it has pressure from near the top so i think it should have cured my problem, thanks again Cam

[camgti 56]

No problem.

 

I had to adjust the thing that pushed the piston inside the booster. It was touching ever so slightly before tightning the MC to the booster. If I had not done this the broke would be dragging. It only needed anout 1 turn or so.

 

Good luck!

 

Cam

[welshpug 1,654]

its possible that just adjusting that rod would have solved things with the 19mm MC, or just bleeding thoroughly again as it cane take a few goes to sort out a multi piston setup.

 

just to add, here's a few surface area comparisons, you'll notice you will now have less outright pressure on the brake pad compared to the 19mm master cylinder, however it will be significantly shorter travel, also you'll have more leverage from the larger brake disc.


2 x 34.6mm opposing pistons = 20.8cm²

 

1.9 205, 48mm single piston (IIRC), 18.08cm²

 

S16 caliper, 54mm, 22.9cm²

 

gti6, 57mm piston. 25.5cm²

 

 

 

19mm MC, 2.8cm²

 

22.2mm MC, 3.8cm²

 

23.8mm MC 4.44cm²

[Pugmanian Devil 1]

Does anyone know what the size of the small spline lock nut on the piston inside the brake booster, the adjuster screw is 7mm metric hex.

 

i tried a 8mm spline spanner which wont go over and a 9mm which goes over it but is slightly too big, my guess is that it must be an imperial size, this is what i want to know.

 

Cheers

[welshpug 1,654]

Just grab hold of it with pliers or mole grips and turn the hex.

[Dizzee stuff 33 1 Cars]

I crushed the end of a 3/8 ring sanner till it fitted just nice, you will proberly need to bend the spanner a fair bit as the nut is a fair way down inside the servo.

[welshpug 1,654]

not if you just push the pedal down

[Pugmanian Devil 1]

Took the car for it's MOT today so got to drive it and feel the brakes... The pedal travel is spot on now using the 23.8 M/C but still lacking in brake effort.

 

The brakes starts to bite the moment you start pressing the pedal and feels smooth up to about halfway'ish on the pedals travel and then it get's really hard and no matter how hard you press it from then onwards it remains hard with little brake effort.

 

I've had a thought that the servo maybe lacking vacuum due to various factors (correct me if i am wrong anybody) I am running a braided line directly from the vacuum pump (XU10J4RS) to servo with just the check valve which goes into the servo (no other valve's in-line)

 

The diameter of the braided line is around 17mm I/D iirc

Does anyone know if the vacuum pump alone is enough vacuum to run the 1.9gti servo without any inlet manifold vacuum as the standard gti6 uses both.... can this be causing my brake pedal to go hard around halfway down??

 

I am going to run some test's tomorrow on the servo, vacuum pump and see how it goes but if someone can verify if the vacuum pump alone should be enough vacuum for the servo or if i should be thinking to tap into the inlet.

The gti6 utilizes some sort of amplifier valve inline plus inlet manifold vacuum.

 

A thought that perhaps the braided line is too big I/D could this be dropping the vacuum pressure??? as the 6 engine uses an assortment of pipe work and valves which are of smaller I/D going into the servo.

 

Edited June 6, 2014 by Pugmanian Devil

[Pugmanian Devil 1]

[Miles 331 1 Cars]

It's fine, if you measure the vacuum of the pump it's around 8 times better than the manifold, Just think of a devil's fuel (Derv) and it's the same pump.

[Pugmanian Devil 1]

If its not vacuum pressure then servo!!?

 

If the servo has smooth movement from the top to bottom on the pedal (with M/C out ofc) is air tight and holds its vacuum can it be the cause to this problem?? i am at a loss with whats happening with the brakes.

 

I thought it could also be the hi spec front calipers so i done a test earlier to see if i could identify if a caliper was holding less than another so i jacked the car up and with the engine off put a brake bar on the pedal as tight as i could then tried to turn each wheel (using good old arm power) in turn front and back and i could turn all 4 of the wheels with the same amount of force applied to each wheel so the pressure to all the brakes is approx the same in effort (took a bit more force to turn the front, but not much more)

 

So i dont feel its a caliper issue...

 

Any ideas anyone

[wicked 103 3 Cars]

If you step in your car, press the pedal a couple of times and then firmly before you start the engine.

Keep it pressed and then start the engine. You should feel a difference in the force you need to apply, If not, the vacuum does nothing.

[Pugmanian Devil 1]

If you step in your car, press the pedal a couple of times and then firmly before you start the engine.

Keep it pressed and then start the engine. You should feel a difference in the force you need to apply, If not, the vacuum does nothing.

if i pump the pedal when the car is off in turn making the pedal go hard, then hold my foot on the pedal while starting the car the pedal does sink slightly once the car starts which indicates that the vacuum is working within the servo.

 

I have ordered a new check valve none the less and am going to measure the vacuum on the gti6's vacuum pump and go from there.

 

Thanks for the feedback Wicked

[Pugmanian Devil 1]

Hi guys,

 

So i done the vacuum test today on the pump and on idle i got a reading of 5 to 10 in-hg, when i gave the engine a rev it shot up to 25 in-hg.

 

I also attached two brake line pressure guages to the front and rear calipers, while the engine was on idle i could only get 500psi on the front and 400psi on the back with the brake pedal feeling rather stiff.

The interesting part is when i hold pressure down on the brake pedal and rev the engine i feel the pedal begin to sink down further (as it should do) vacuum assist begins to work and got the front line pressure up to 900psi and the rear to 700psi.

 

After such testing i strongly feel i have a lack of vacuum which is causing all my problems regards to line pressure and stiff pedal (Not Servo Problem)

 

What i ask is if anyone can do a vacuum test on a gti6 vacuum pump on idle and while revving and let me know the results in order to compare my results, otherwise i think i will ditch the pump altogether, fit a blanking plate and run the vacuum straight off the inlet.

 

I have not done this yet because it means i need to remove the inlet, drill and tap it and attach a fitting then vacuum hose to it, so before i do that i would appreciate if someone can tell me either what these vacuum pumps should pull on idle or a reading from a working car that just uses the pump for the source of vacuum for the servo.

 

The engines on throttle bodies and a modded inlet which has no connection for a vacuum hose... (yet)

Edited June 12, 2014 by Pugmanian Devil