Author Topic: Advice on strange brake pedal feel  (Read 508 times)

Offline andrewrx7

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Advice on strange brake pedal feel
« on: January 02, 2015, 04:44:01 PM »
Hi,
I'm after a bit of technical advice, if anyone can help. It is a question about brakes but not on a rotary car, but not that that really matters.
One of the cars I have, seems to have a delayed action on the brakes after I press the pedal. By this, I mean that if I press the pedal, even very hard, there seems to be a couple of seconds delay before I feel any braking effect. This is also the case if I emergency stop. Discs are all good, virtually new condition, almost new pads all round, fully bled the system with new fluid. I've checked all four calipers, and carriers and slider pins are all free to move. Pistons seem to move ok. No fluid leaks anywhere.
So why the scary feeling brakes? There seems to be no initial bite until the pedal is pressed pretty hard even then it feels like there is little braking effort. If I emergency stop the car just slows down rather than the ABS kicking in as with the other cars. It is just very disconcerting pressing the pedal and waiting for the brakes to apply...!
Any ideas what could cause this? I've bled the system several times, never got any air from any of the calipers. I keep thinking a master cylinder issue, so do these symptoms sound like it could be?
I'd appreciate any pointers of what may be wrong!
Thanks
Andrew


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Re: Advice on strange brake pedal feel
« Reply #1 on: January 02, 2015, 05:03:20 PM »
Does the brake pedal slowly descend to the stop? Do it with and without running.
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Offline andrewrx7

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Re: Advice on strange brake pedal feel
« Reply #2 on: January 02, 2015, 06:29:06 PM »
The pedal does have some compression when pressed and does move a little if I keep the pressure on, but then it is the same for the other cars I drive. Maybe the pedal does feel a bit softer, but it doesn't go to the floor if, for example, there were a leak releasing pressure in the hydraulic line. When I press the pedal, and quite hard, the pedal feels ok, but there is just this lack of response from the brakes - there is no bite or grip, then something tends to happen. From my own description, I'd say it were sticky pistons in the calipers, taking time push out against the discs. But having put new pads on the front, the pistons in both calipers pushed back relatively easily. Scratching my head, to be honest, about this!

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Re: Advice on strange brake pedal feel
« Reply #3 on: January 02, 2015, 06:55:12 PM »
what make of car? standard brakes? prob best to get it on an mot brake tester.
92 FD rx7 Twins 13.5psi. Track car
94 Westfield Se 231 rx8 street port powered. Race car
EAT SLEEP ROTOR REPEAT.

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Re: Advice on strange brake pedal feel
« Reply #4 on: January 02, 2015, 07:32:09 PM »
Sounds like servo....check the hose from servo to engine .
92 FD rx7 Twins 13.5psi. Track car
94 Westfield Se 231 rx8 street port powered. Race car
EAT SLEEP ROTOR REPEAT.

Offline rote8

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Re: Advice on strange brake pedal feel
« Reply #5 on: January 02, 2015, 08:57:17 PM »
^^^^^^ What he said, sounds like lack of servo assist. Sitting stationary and pumping the pedal does it get harder. The pedal I mean
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Offline andrewrx7

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Re: Advice on strange brake pedal feel
« Reply #6 on: January 03, 2015, 07:22:00 AM »
Thanks for the replies. I was wondering servo, but thought that if the servo was not working then the problem would be the opposite...with the pedal very hard but with a lack of brake effect (I am thinking back to old Polo's for example). I wonder about popping off the vacuum hose to the servo unit and see what happens...not necessarily on the road, by the way!
If I double or triple press the pedal in quick succession, the pedal is harder, but if I keep the pressure on the pedal will slowly move.
The car is an MG ZS 180. Oddly,  the brakes work well on the track, when hot. I had ebc yellows stuff pads in which are good when hot and used hard, and I initiallythought these were the problem, glazed and cold for the road. But putting a new set of oem equivalent Mintex in and the normal road driving is still unchanged, as described in my posts, so not the pads.
So, I was wondering whether anyone would say 'yes the problem is this' from the description, so I don't end up buying new parts that are not needed.

Thanks again!

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Re: Advice on strange brake pedal feel
« Reply #7 on: January 03, 2015, 08:57:44 AM »
Think the best thing is to get on a mot brake tester. This will tell you brake force for each wheel bind etc.
 Also being a track car you couldve melted the grease in the front sliders. When you changed the pads did the carriers move freely?
92 FD rx7 Twins 13.5psi. Track car
94 Westfield Se 231 rx8 street port powered. Race car
EAT SLEEP ROTOR REPEAT.

Offline andrewrx7

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Re: Advice on strange brake pedal feel
« Reply #8 on: January 03, 2015, 09:25:20 AM »
Sliding pins move freely, definitely checked those when I did the front pad change. Rears were seized but I replaed the pins and repacked with grease about 8 weeks ago so they are also good. Thanks for the suggestion.

I have read that when pushing back pistons into calipers you should really vent the brake fluid via the bleed nipple so as not to force fluid the wrong way through the master cylinder, since this can cause a seal to flip or unseat and hence leak during use causing a lack of hydraulic pressure. Not sure how true this is though!

What about the rubber flexible hoses? Do these "relax" over time and allow some bulging hence loss of pressure upon initial press of the pedal, or are they more "all or nothing", by which I mean they work ok or catastrophically fail? Would a braided kit actually do some good? Could the heat from several track days have caused the rubber hoses to weaken? Just another option (right or wrong) into the mixing pot!

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Re: Advice on strange brake pedal feel
« Reply #9 on: January 03, 2015, 09:44:11 AM »
You are supposed to open bled nipple but ive changed hundreds of pads and just push the caliper back. Braided lines on a track car are a very good idea for pedal feel and standard brake holes have been known to delaminate inside createing a blockage. Id get on a brake tester now get some numbers to work with plus an mot tester will be able to diag for you.
92 FD rx7 Twins 13.5psi. Track car
94 Westfield Se 231 rx8 street port powered. Race car
EAT SLEEP ROTOR REPEAT.

Offline andrewrx7

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Re: Advice on strange brake pedal feel
« Reply #10 on: January 03, 2015, 12:59:50 PM »
Cheers. Got an mot testing garage near to where I work so hopefully they will be willing to help when needed. For £60-70 I may just invest in a braided hose kit.

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Re: Advice on strange brake pedal feel
« Reply #11 on: January 03, 2015, 03:40:37 PM »
Braided brake lines are awesome but i'd investigate/fix the issue first.
92 FD rx7 Twins 13.5psi. Track car
94 Westfield Se 231 rx8 street port powered. Race car
EAT SLEEP ROTOR REPEAT.

Offline andrewrx7

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Re: Advice on strange brake pedal feel
« Reply #12 on: January 03, 2015, 04:35:22 PM »
but i'd investigate/fix the issue first.

Indeed and agree totally, but I wonder whether heat from 4 track days (well, 1 day and 3 half days to be more accurate!) in the last 6 months has caused deterioriation in the rubber lines making them a bit more flexible than they should be? The car is almost 12 years old now, so could these lines be the issue? Just eliminating possibilities as I go along and this is at least a place to start!

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Re: Advice on strange brake pedal feel
« Reply #13 on: January 03, 2015, 06:31:51 PM »
Yep see what saying and a good excuse to get some decent brake fluid in there...i can recommend endless has the 2nd highest boiling point only just been knocked off top spot.
92 FD rx7 Twins 13.5psi. Track car
94 Westfield Se 231 rx8 street port powered. Race car
EAT SLEEP ROTOR REPEAT.

Offline andrewrx7

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Re: Advice on strange brake pedal feel
« Reply #14 on: January 03, 2015, 08:59:27 PM »
I use ATE Super Blue, which seems to have a very good rep too, with a boiling spec just under the Endless (RF 650), having just looked. But then it is a fraction of the cost to! Wow, the Endless is pricey, but then if it works under extremes on the track, then the cost can to be justified.