What IS Going On With My Brakes?!?

[Mj17]

Hi all,
What do you think is wrong with my brakes?

Basically the pedal is all over the place.  Sometimes it's rock hard and right at the top of the travel but the next time you press it it can be softer and half way down the travel.  Stand on the pedal and it always works, keep pressure on it and it just stays there (doesn't drop at all).

Brake pipes are new (copper/braded flexible hoses approx. 12 months/10k ago), rear wheel cylinders are new (approx. 6 months/5k ago), front calapers are new (recon from Big Red approx. 18months/15k ago), fluid is no more than 12 months old.

I've tried bleeding and re-bleeding using an Ezebleed and (other than the first time) all I get is clean, bubble-free fluid from all 4 corners.

I try and watch the fluid level at least once a week and it's been fine for ages but had dropped when I checked on the weekend.  I've been around and reassembled one of the rear drums as it wasn't self-adjusting and given each corner a bleed - no signs of air anywhere, nor any signs of leaking fluid.  No signs of leaks from the master cylinder in the engine bay.

So...can fluid leak past the m/c seals and from there get sucked through the engine and burnt (explaning the loss of fluid without a trail) or...?

Thanks for any help because I'm running out of ideas of things to try! :)

[GreenV8Machine]

do you have a servo?

Im having issues with my brakes at the moment, press hard and just before the car stops the pedal goes soft and brakes disengage!

[CRAJ]

Do the pedal woes happen after lots of steering lock input, Ibelieve this is called pad knockout but is also said to be caused by worn stub axles allowing thge disc to 'wobble' without the calipers, hence pushing the pads and pistons in and giving you a brake pedal akin to when you have just replaced the pads!

Cheers Colin.

[CRAJ]

Do the pedal woes happen after lots of steering lock input, Ibelieve this is called pad knockout but is also said to be caused by worn stub axles allowing thge disc to 'wobble' without the calipers, hence pushing the pads and pistons in and giving you a brake pedal akin to when you have just replaced the pads!

Cheers Colin.

[Russell]

Sounds like you could have a faulty master cylinder. If you press on the brakes really slowly (with the car not running), does the pedal gradually move to the floor?

I've got a similar issue with the TR6 at the moment, but if I stamp on the brakes it stands on it's nose. No fluid loss with mine though.

Russell.

[Mj17]

1) Yes, have a servo but I wouldn't still be driving it if the pedal ever went soft and the brakes came off! :o

2) No, doesn't seem to be related to steering input.

First thing in the morning I've got to make a couple of full-lock turns to get out of my parking space and (certainly for the first two presses - can't remember the third/etc) it's rock solid and at the top of the travel.

In contrast driving home last night I pulled off the motorway on to the slip lane and when stopping for the roundabout it was half way down.

3) Will give it a go with the engine off and see what happens...

[Mj17]

1) Yes, have a servo but I wouldn't still be driving it if the pedal ever went soft and the brakes came off! :o

2) No, doesn't seem to be related to steering input.

First thing in the morning I've got to make a couple of full-lock turns to get out of my parking space and (certainly for the first two presses - can't remember the third/etc) it's rock solid and at the top of the travel.

In contrast driving home last night I pulled off the motorway on to the slip lane and when stopping for the roundabout it was half way down.

3) Will give it a go with the engine off and see what happens...

[GreenV8Machine]

When they come off, repress the pedal and they come back hard.

[AlanChatterton]

This is definately pad "knock out"........ as suggestd by Colin (Craj!).......... we learned about this at the weekend from one of the Triumph "gods".........!!

What is happening is that due to either worn front wheel bearings, or worn front stub axles, is that is you go round a corner, the whole disc/hub assembly is moving in and out a bit, which in turn pushes the pads further back into the caliper than they should.

The next time you press the brakes, the pedal drops alarmlingly as you have to move the pads 3 times as far as normal....... just like when you fit new pads, you need to pump the brakes a few times to get the pads in contact with the discs again......

With Stag brakes (like I have) and with the worn stub axles (like I have too!!) the problem is amplified as the discs move more due to them being bigger.......

either, get used to doing a "double-tap" on the brakes..... as I do now, or replace the stub axles with Stag units and put new wheel bearings in and the problem will be solved.

Hope this helps!!

[beefertoo]

could be nothing more than rear shoes want adjusting up, try pulling the handbrake on and seeing what happens to the pedal. my tolly had the same problem

craig

[Roger]

SOUNDS MORE LIKE A SERVO PROBLEM,  CHECK IT THIS WAY. SWITCH OFF ENGINE PUMP BRAKE PEDAL A FEW TIMES UNTIL IT BECOMES HARD THEN HOLD FOOT ON BRAKE PEDAL & START ENGINE, THE PEDAL SHOULD GO DOWN A BIT IF IT DOESN'T THEN THE SERVO IS SHOT.

[marc1estate]

Could be fluid bypass if you're losing fluid without a trail, take the master cylinder off, stick a finger or other impliment into the servo to see if any fluid is in there, I had this years ago on LFH and only figured out what the problem was when the fluid level in the servo got so high that it got sucked into the cylinders and burned off, looked like a steam train  ;D

[Mj17]

Thanks for the suggestions - a few things for me to try out.

My feeling is that this isn't so simple as pad knock-back as I've tried the pedal pumping, thinking it was air in the system. If the pedal is half way down and I pump 3 or 4 times...it's still half way down on the 5th press.

[Lord Sorbington]

Is the servo arrangement on a mk2 PI the same as on a 2000?  If so, as has been stated, if you're losing brake fluid with no signs of a leak then it's probably the master cylinder leaking into the servo.  I had this problem on mine.  One exchange master cylinder from Wins later (other suppliers may be available, although not when I was looking) and it was sorted.

The brakes felt a lot better with the new master cylinder too, the pedal changed from being rock hard to nice and progressive.

[Mj17]

OK, pumping the brakes while sat at the lights doesn't change the the pedal position (does confuse the driver behind though).

Pumping till (the pedal is) hard with the engine off, then starting the engine with food on brakes leads to the pedal dropping ~2" (about a shoe toe's height from what I can tell).  It then stays there if I maintain the same pressure on the pedal.

Sounds like time to order a set of m/c seals (if I'm going to take the m/c off I might as well change them anyway while it's off the car) and have a poke in the servo...

[Lord Sorbington]

It's also worth a very close check of your flexi hoses & pipework for damage.

Years ago, my parents had a Maxi on which the brake pedal would occasionally go to the floor without bringing about any retardation.  It would be back to normal immediately afterwards.  It transpired that one of the flexi hoses had a tiny pin hole, no obvious leak but it caused a few scary moments.

[CharlieB]

Had the same problem on my Spitfire, it was simply the piston sticking in the master cylinder & not fully returning when I took my foot off the pedal so next time it was already half way down.

[Dizzy]

I think Russell had it right in suggesting trouble with the master cylinder.  The suggestion of "pad knock-out" needs to be forgotten as I feel it is more imaginary than real (I've never come across it in 40 years of running and working on a multitude of disc-braked cars).

If the problem is the secondary cup (i.e. the seal separating the master cylinder from the servo), then the fluid level would have dropped due to fluid entering the servo.  If it is the main cup, fluid would not be lost, as follows.....

When the main cup is at fault, it doesn't open quickly enough to seal as the piston goes down the bore (think "bicycle pump") so the brakes don't come on straight away.  No fluid is lost, the cup simply moving through the fluid and then back again as the pedal is released - just like a faulty cup in a bicycle pump.  Often, pressing the brake pedal sharply will expand the cup straight away and give a good brake, whereas pressing the pedal very gently will see it go a long way down before you get any resistance.

[Mj17]

Dizzy wrote:

The suggestion of "pad knock-out" needs to be forgotten as I feel it is more imaginary than real (I've never come across it in 40 years of running and working on a multitude of disc-braked cars).

I experienced the results after replacing the brake pade and moving my Spitfire without pumping the pedal first - that one required a quick change of underwear ;)

[heraldcoupe]

Dizzy wrote:

The suggestion of "pad knock-out" needs to be forgotten as I feel it is more imaginary than real (I've never come across it in 40 years of running and working on a multitude of disc-braked cars).

Pad knock-out is very real, and quite common in my experience.
Cheers,
Bill.

[Nick Jones]

I agree pad knock-out is real (certainly happens on Vitesses and Audi A6s), but I also don't think it's the problem here.

Almost certainly a knackered master cylinder with any leakage going into the servo can.

Nick

[Lord Sorbington]

Mj17 wrote:

I experienced the results after replacing the brake pade and moving my Spitfire without pumping the pedal first - that one required a quick change of underwear ;)

It's particularly important to remember this if you're lucky enough to own a vehicle with automatic transmission!

Must say that I've never, ever encountered pad knock back on any car, and I've owned some right old nails!

[Deleted User]

sorbs wrote:

..... and I've owned some right old nails!

like that silver heap that you picked up today (evil)

[shedmonkey]

do a couple of alpine passes as fast as you dare  brake fade combined with pad knock out adds to the fun! I first had it on my 2500TC during the London Lisbon rally  in 2000 -confused me at first thinking I had a terminal fault,worth making sure your rears are adjusted correctly or the self non adjusters are working the best they can, got the GT6 to do it on a few rallies as well - I guess as things get hotter than normal the bearing play could increase?

[Dizzy]

The pads will always be pushed back a bit due to disc run-out, bearing play, etc, but surely never to the extent that the brake pedal sometimes goes halfway down before resistance is met!  This is the problem Mj17 is faced with and, it seems, it is happening in everyday driving, not in rallying or circuit racing.

I think it is most unhelpful for someone to take a wild stab in the dark at the source of a problem and declare it as "definite".  Nothing is definite when fault-tracing until the actual fault is found, and jumping to a conclusion before eliminating the common causes can lead to a great deal of unnecessary work and expense.  I know this for a fact (i.e. it is a "definite"!!) as I went there many times in my earlier days!