Brake problems, absolutely no feel and cant stop the car

Brake problems, absolutely no feel and cant stop the car

Author
Discussion

DBaldwin

Original Poster:

126 posts

175 months

Sunday 24th June 2018
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Hi guys, looking for some help regarding my brakes as im at a bit of a loss.

I’ve got a heavily modified clio 172 race car which has:
a tilton pedalbox with independent front and rear master cylinders,
new copper pipes with a tilton rear bias adjuster,
Standard rear brakes (new)
Standard front calliper with carbon Lorraine rc6 pads, ktec discs and Castrol react fluid

It’s all newly built and i feel ive ticked all the boxes to make up a great set up and i had most of the work professionally done yet im having issues of having absolutely no feel through the brakes and they get progressively worse to the point of dangerous through the period of a 15 minute race as i just cant stop the car. No amount of brake bleeding seems to solve the issue and also gives no indication of any masses of air in the system.

The front pads did however look heavily worn which i thought was premature after just 2 or 3 days on track? The rear brakes look fine. It’s a similar set up to my competitors apart from the pedalbox set up.

This leads me on to whats causing the awful pedal feel and it to be spongey and generally crap? I’m going to replace the front calliper as a matter of eliminating them as their virtually the only thing that isn’t new but i have a feeling the problem must be hydraulic. Any ideas appreciated as a bit lost as to what it could be.

frodo_monkey

670 posts

195 months

Sunday 24th June 2018
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What size front and rear master cylinders?

DBaldwin

Original Poster:

126 posts

175 months

Sunday 24th June 2018
quotequote all
frodo_monkey said:
What size front and rear master cylinders?
Stupidly forgot to write down what they were yesterday when i took the engine out, will note them down tomorrow. Supplier recommends 0.625” front and 0.700” rear so hoping thats whats fitted.

AdeRacing

31 posts

69 months

Sunday 24th June 2018
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Shouldn't really need a rear bias adjuster. The bias adjuster on the pedal box should be enough.

Where do you have the pedal ratio set? If it's at the bottom of the range then you would have to put a lot of effort in. Have you run with a non-servo setup before? It takes some getting used to if you haven't.

I have the same pads and pedal box in my car and the brakes are incredible. The CL pads do wear fast, that's the nature of them,

How did you bleed the brakes? With a bias box you have to bleed the front and rear on each side at the same time. Not each corner as you would with a standard setup.

anonymous-user

53 months

Sunday 24th June 2018
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It doesn't matter which sides you do, all you need is to have one front and one rear open at the same time, so you get a full stroke on the cylinder.

You could just crack one rear open and then do both fronts, before then leaving one front open and doing both rears. Make sure you are bleeding them properly by locking off the bleed nipples when the pedal is at the bottom of the stroke, otherwise you will drag air back into the system.

Get rid of the bias valve in the rear line, the only ones i've found work are those made by AP Racing. With a bias pedal box you don't need a rear bias valve, you set the balance on the master cylinder sizing and bias bar.

Lots of none standard pedal boxes in saloon cars are utter crap compared to the stock setup, quite often the pedal geometry is all wrong. It's easy to cock it up.

Once you have the system bled properly and the master cylinder sizing correct, you need to make sure when you press hard on the pedal the bias bar is 90 degrees to the pushrods. This will usually mean when you release the pedal the bais bar will not be at 90 degrees to the pushrods.

The other issue is making sure the pedal isn't going over centre when pressure is applied. If the pedal goes over centre you get no brake feel.

Finally, check your master cylinder caps are breathing properly, they should be open to atmosphere via the little breather hole in the cap top, i've seen it where the cap isn't breathing, you then don't get the master cylinder filling properly on a stroke. You can use baffles in the master cylinder pots to stop any spillage issues through the breather, it's easy for that setup to not work and give the same issue as a cap not breathing, so if you have those baffles, take them out and see if that improves the brakes.

It's worth checking the pedal ratio is suitable for the cylinder sizes. Typical is 5:1 to 7:1, the smaller the ratio the harder you have to push, the larger the ratio the longer the pedal stroke will be.

andygo

6,786 posts

254 months

Tuesday 23rd October 2018
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Glazed front pads. Swap them, especially if they are worn.

Bertrum

466 posts

222 months

Wednesday 24th October 2018
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Sounds like pad knock off?

Have you tried lightly applying the brakes on the straight prior to applying them in full to re-set the pads back to the disc? I would try that first.

This issue is common on MK3 MX5's when you remove the ABS valve.







Edited by Bertrum on Wednesday 24th October 11:59

Drumroll

3,738 posts

119 months

Wednesday 24th October 2018
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"boiled" brake fluid or air in the brakes?

MrJingles705

409 posts

142 months

Wednesday 24th October 2018
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Drumroll said:
"boiled" brake fluid or air in the brakes?
I was wondering about brake fluid boiling as well, since it's getting progressively worse.

OP, do the brakes "come back" a little (but not all the way) once the car has been off track and cooled?

Jerry Can

4,422 posts

222 months

Saturday 27th October 2018
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I think it might be something to do with the Clio 172. I raced with a friend who had one a couple of years ago and whilst it did not have a custom pedal box, the abs was removed and the feel was awful. basically any hard press on the brakes locked them up meaning you could only use the first couple of cm of travel, meaning longer braking zones, it was costing at least 3 secs a lap. I would suggest in your case you reinstall the abs system, or ensure that the MC is matched to non abs use. calliper bore size, mc size have an impact on each other so check those

tapkaJohnD

1,930 posts

203 months

Friday 9th November 2018
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"No feel, and no grip"
On a car modded for racing.

I'd say your mechanics have fitted pads too 'hot' for the car, that never get up to temperature.
Try fitting a set of road pads for the front , to minimise cost, and taking it for a test. You should get grip, that then fades as the road pads overheat. They will come back as they cool down again, but won't ever be as good after overheating.

if that conforms the diagnosis, then fit hotter, but not 'Full Race' pads.

John
PS do not forget to bed in the test or new hotter p[ads!

BertBert

18,953 posts

210 months

Friday 9th November 2018
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Would they wear so quickly though?

The Wookie

13,909 posts

227 months

Sunday 11th November 2018
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Don’t know if the OP got sorted or not but anyway...

Sizing seems about right, only a light car so I’d be surprised if it was a raw thermal capacity issue and CL’s are decent pads

Definitely worth rebuilding/replacing calipers, if they’re dragging that will boil the fluid and wear the pads rapidly

Have you adjusted the master cylinder lengths correctly? If they aren’t fully extended then you may have partially or fully closed off the breather which will mean the fluid has nowhere to go as everything expands... although this usually results in the car grinding to a smoking halt in my experience

Also, conventional wisdom suggests that it’s not worth having a prop valve if you have adjustable bias at the pedal (as a brake engineer I agree as they’re inherently inefficient), however as an FWD racer they are useful as the rear brakes do so little. I find a prop valve a useful tool to adjust the lockup characteristics and then use the pedal bias adjuster to tune the trail brake characteristics. Indeed every BTCC car has both.