951 Brake Bleeding Frustration

[Rob]

I've had 944s and 951s (and a 968) since 2002 and have never had a problem getting a good "pedal" until now, with my 1990 951.  I changed the fluid (DOT4) before my son did a track day at QR.  The brakes worked brilliantly throughout the day, but my son reported a drop in pedal height toward the end of the day.  We figured it would recover with a fluid change.  After a full fluid change using a pressure bleeder, pedal height with the motor NOT running was good, with a ~10mm free play as per spec.  However, on starting the motor and the vacuum booster coming into play, the pedal sinks probably another 20mm with moderate foot pressure.  Redid the bleeding and the problem persists.  BTW the car has braided steel lines throughout.

I recall reading (somewhere on line) that there are some "tricks" to getting a good pedal, although I haven't needed any "tricks" previously.  I'm tending to think there may be an issue with the master cylinder, but before I rush to the keyboard and buy one, I'd appreciate any insight others might have on the subject.

Thanks in anticipation

[Peter M]

Rob,

Does it have ABS?

[Airhead]

Check your pads. They could be down to 'bugger all' so a long pedal will be the result.

[Buchanan Automotive]

If you have noticed a change & obviously you have , and you know how to bleed the system & simple things like pads & disc's OK ( basic stuff ) & there are no brake fluid leaks / wet areas anywhere & you do not have excessive wheel play , meaning a lot of wheel bearing play ( F or R )that when moving , the forward back movement of the wheel bearing/s will push the pads back ( via the disc) a little or a lot 

Now with all that said and and done then there is only one thing remaining & this thing will normally be OK ( if the brake fluid is changed every two years as a road car only ) around 10 to 15 years on average , some times in exceptional circumstances maybe 20 years ( rare ) and that thing is the brake master cylinder 

How old do you think the master cylinder is ? , could it be 25 years old ?

If it is I would not hesitate to get rid of it on age alone 

Regards
Bruce Buchanan
Buchanan Automotive

[Rob]

Gents, thanks for your responses.

Peter M - yes - it has ABS

Airhead - always conscious of pad wear - these are less than 1/3 worn.

Bruce - checked my records, going back to new in 1990.  The car has been tracked by 3 previous owners, plus me, with (according to the receipts from Weltmeister and Peter Fitzgerald Racing) no expense has been spared.   But, despite pad, disc and fluid changes on about a yearly basis, no mention of a master cylinder change.  It has 211,000km on it.  So - it looks like the M/C is the original, so probably worth replacement.  Just about everything else in the car has been replaced (largely by the POs, fortunately), so we might as well go for completeness...

Bruce, I'm sure we all are appreciating your input to the forum.  The sad thing is that you only have the time to do so because you're recovering from a hip (?) replacement, so I guess we'll have to make the most of your recovery time while it lasts.  Hoping (for you) for a speedy and complete recovery.

Regards

Rob

[Peter M]

Rob,

I'm no expert on this but had a similar experience recently with a Mitsubishi Delica that would hold firm pedal pressure fine without the engine running but as soon as you started it up the pedal would slowly sink to the floor!  Brakes worked fine despite this.  The only thing I could figure out was it appeared the ABS unit was bleeding down the line pressure.

So what I'm suggesting is that your master cylinder is fine and it's related to the ABS.  What happens if you pull the ABS fuse and then start the engine to provide vacuum assistance?

Please report back when you have nutted this out as I'm interested!

Good luck

Peter

[Rob]

Peter

Just got back from the shed after trying your suggestion.

Unfortunately no cigar, but certainly worth a go.  Thanks for suggesting.  Looks like a new M/C $$$$!

BTW, how's that 998 going?  Scared yourself yet?

Cheers

Rob

[Buchanan Automotive]

Hi Rob , thanks for you're kind words , in regards to the 1990 year type 951 , if it was just a road car ( meaning no track events ) a 25 year old brake master cylinder is still a mild concern & it is true in 90 % of the time ( when the master cylinder is showing classic" I am dying")  the brake pedal will start to sink away from under you're foot , naturally this is when one is holding the car on an incline and with a steady pedal force , but under sudden braking force the pedal may well feel normal , at least its a good warning sign

Or the other 10% of the time you can not get a decent brake pedal after bleeding a 25 year old master cylinder & nothing will bring it back because of age & internal wear 

Now a 25 year old Master cylinder on a car with lower than normal engine compression ( 951 , 930 , 931 ) are all in the same boat , low compression = lower than normal engine braking that one would get from a high compression NA engine , so the dependancy on better than normal brakes on a Turbo car is increased( a bit like a 2 stroke engine in lack of engine braking ) ,and in the case of some track driving it becomes even more important , so for safety sake alone a new Master Cylinder is something you do no matter what just to tick that box , it could very well save a lot of money 

Note }   With the use of normal Dot 4 brake fluid at the track , you will get micro bubbles forming in the front brake callipers mainly & thats why we switched to the Castrol SRF fluid around 10 years ago , it saved us removing wheels at / during Super Sprint event days & bleeding the brakes , with the SRF , the fluid says in all year ( multiple Supersprints ) with only micro bleeds the day before in the preparation for the event 

The SRF costs a lot more , but the time saved in NOT having to rebelled at the events save a huge amount of time = money

Note 2 }  Be very mindful of wheel bearings on these cars , we generally repack the front wheel bearings every 2nd Supersprint + replace grease seals  & depending on the Supersprint event it self , if it was hard on the brakes that day( extreme heat ) , then we will dismantle the front wheel bearings more often , you will be surprised on the damage that is done to these bearings under fast track conditions 

  Note 3 }           Rear Wheel bearings }    On Sean's 3.0L 16V 951 road & track car , we replace the rear wheel bearings at the start of EVERY year , , because I am amazed at how many rear wheel bearing failures or near failures we see at our PCNSW supersprints each year & when the rears wear to the pint that you can hear them on "R" spec tyres ITS WAY too late and damage to the forged centre axle ( that goes through the middle is guaranteed )  = very expensive & avoidable 

Where as on road cars , the above is not needed anywhere near that level  , gong to the track beats the crap out of all cars , no matter who its made by

Note 4 }       Drive Shafts }   This has been an ongoing issue we have to deal with on these cars as we put more & more power through them , as an interim , make sure the CV joints are packed with new CV before each event & double check the tightness of the CV bolts 
                        I could go on for hours on this subject , but you get the point 

Regards
Bruce Buchanan
Buchanan Automotive
 

[Rob]

Hi Bruce

Thanks for the advice - I'll order a new M/C and report back in a while.

Regards

Rob

[Timm]

I can confirm what Bruce says about the Castrol SRF, that stuff is absolutely awesome! The first couple track days I boiled my brakes every time towards the end, even with fresh bleeds using ATE super blue. I switched to SRF and the peddle was firm even after a whole sprint day. Yes, it is very expensive, but in the long run it costs about the same as doing multiple bleeds with cheaper fluids. Not to mention the price you pay if you boil your brakes at the wrong time and place (eg before turn 1 at QR, or anywhere at Lakeside).

[Timm]

Another thing that makes the peddle feel spongy under firm pressure is firewall flex causing the master cylinder to move. On my car the master cylinder would move almost a centimeter or so, translating to about the same amount of movement in the peddle.

I'm not sure if it's due to a crack in my firewall, as I haven't gotten around to removing the master cylinder and brake booster to check. However I did make a very rudimentary brace that reduced the master cylinders movement by about half.

Now that the engine is out, I will look into making up a proper brace, that can be tensioned with a threaded rod, similar to this: http://i45.tinypic.com/35a0qys.jpg

 

[Rob]

Hi Timm

Thanks for that.  Although I've seen numerous mention of firewall flex in the US forums, I'd thought it might not be a problem with the RHD cars because of the different force distribution caused by the different pedal location and force transfer bar.  Obviously not, at least in your case.  I've just checked mine and it's as steady as a rock .  Re SRF, I'll put in a new M/C and get that as good as possible with DOT 4 before going to SRF.  I do note from my receipts that SRF has been used by some of the POs.

Cheers

Rob

[Peter M]

BTW, how's that 998 going?  Scared yourself yet?

Wonderful thanks!

Did my first track day in 12 years a couple of weeks ago and was impressed.  Not GSXR fast but that OK, I'll leave that to the talented youngsters nowadays.  In fact in the market to buy a cheaper Ducati just to use as a track bike so I'm pondering a SS1000, 749 or uneconomic repair write off 848 for that purpose.  (Yeah, I know sensibly an already setup R6 or GSXR600 would be cheaper and faster.)  

 

How's that lovely SS of yours treating you?  

[Harvs11]

(when the master cylinder is showing classic" I am dying")  the brake pedal will start to sink away from under you're foot , naturally this is when one is holding the car on an incline and with a steady pedal force , but under sudden braking force the pedal may well feel normal , at least its a good warning sign

Or the other 10% of the time you can not get a decent brake pedal after bleeding a 25 year old master cylinder & nothing will bring it back because of age & internal wear 


Regards
Bruce Buchanan
Buchanan Automotive

these were the exact symptoms with my 30+ year old 911 when I got it.  Replaced the m/c with a new unit and now it's a solid pedal all day. 

 

Btw. Slightly off topic,  why do 911 brake fluid reservoirs have a overflow vent pipe?  I've never seen this on any other car.  

[Rob]

.

How's that lovely SS of yours treating you?  

Great thanks, pampered with Igniitech programmable ignition, Factory Pro jet kit, cored 900FE hi-mount mufflers, custom top fairing...

these were the exact symptoms with my 30+ year old 911 when I got it.  Replaced the m/c with a new unit and now it's a solid pedal all day. 

 

Btw. Slightly off topic,  why do 911 brake fluid reservoirs have a overflow vent pipe?  I've never seen this on any other car.  

Thanks Harvs, I hope that fixes the problem.

Re the 911 reservoir, not sure what you mean, but the 944 reservoirs have a blanked-off pipe that looks like an overflow, to run to the clutch M/C on LHD models.

[Harvs11]

Great thanks, pampered with Igniitech programmable ignition, Factory Pro jet kit, cored 900FE hi-mount mufflers, custom top fairing...

Thanks Harvs, I hope that fixes the problem.

Re the 911 reservoir, not sure what you mean, but the 944 reservoirs have a blanked-off pipe that looks like an overflow, to run to the clutch M/C on LHD models.

911 SC reservoir has a hose that drains to ground just behind the front bumper, left side. I've routed it to a catch bottle to stop it ruining paint.  After a day of heavy braking it loses an amount of fluid and needs to be topped up. 

[Buchanan Automotive]

Because a 911SC / 3.2 911 / 3.3 Turbo etc has its brake master cylinder & more important the fluid reservoir in the front luggage compartment ( where luggage goes + the luggage area carpets ), if the fluid level was unintentionally a little too high and the brake system heats up quite a lot or if someone put new brake pads in ( without first draining some reservoir fluid ) , well with a high fluid level to start with, the fluid that is being forced back into the reseviour has to go somewhere and it will naturally weep/seep out of the cap breather and this excess spilt fluid is now free to splash all over the luggage & or carpets etc , hence why these cars had a overflow brake fluid drain away from the car before the fluid in the reservoir fluid level reaches the height of the cap breather

Where as the brake master cyinder/ reservoir on a 944/ 951 / 968 / 928 etc  is in the engine bay & any overflow / spillage of brake fluid will cause no damage to carpets or luggage .

Regards
Bruce Buchanan
Buchanan Automotive

[Rob]

Well, just put in a new ATE master cylinder, bled brakes and went for a short test drive.  Still the same!  Car still stops like it's run into a brick wall, but the pedal travel remains more than I think is acceptable.  Going to look at the backs of pads & calipers next.  To clarify that, the pedal feel is as if there's a layer of rubber on the backs of the pads.  I know that's not the case, but I'm going to look anyway...

[Buchanan Automotive]

Good news is the 26 year old master cylinder is gone ( far too old to go anywhere near a track with that old relic ) , now think back to the track day , was there a sudden change during the day in the pedal feel ? ( I think you mentioned that ) , if so then something did in fact change 

Now first of all , it is always a bit difficult to properly bleed a new master cylinder , so on the road test , did the brake pedal feel ok "ish" if ( while driving ) pumped it up quickly & then leaving the brake pedal alone for 30 seconds ( still while driving ) you then just did one mild application and straight away it had more pedal travel , but you were able to pump it up a little ( rapid little pumps ) & it felt better for a few seconds after that

If so then there is still micro bubbles in the system , if no then }

Remove the master cylinder & measure the distance the push rod ( from within the brake booster is at in the rest position ) , then calculate the distance the position in the centre of the concave master cylinder piston & then calculate if there is an air gap , meaning the push rod should almost touch into position into the concave M/ cyl piston back , but its not to push the piston in at all when bolted up

So in other words if you have ( say 3mm ) gap this gap has to made up by the pushing the brake pedal further , there should be a tiny clearance gap of say 1/2 mm ( half )

Also , make sure you're pedal box ( where the pedals go onto ) is not broken away from fire wall roof or cracked and causing free play at the pedal area ( not common ) but worth a look 

PS } you are bleeding all 8 bleed nipples on the 4 brake callipers ( 2 per calliper ) , I remember some years ago talking to an owner who bleed his brakes ( 928S4 ) and he had issues with pedal feel until he found out there were two bleed nipples per calliper 

Regards

Bruce Buchanan
Buchanan Automotive

[Rob]

Thanks Bruce - I hope you'll be able to find a little time in future after tomorrow, especially for us neglected 944 owners...

Re your road test question, although I did do some experimentation (limited by afternoon traffic) and don't believe the pedal pumps up at all once the motor is running - that is, there are two pedal positions, one when the engine is off and another, lower position, when the motor is running.  Pedal pumping doesn't seem to alter either position.

I'll do some more road testing, checking as you advise, and see what happens.

Re bleeding - yes, I'm well aware of the nipple count and have read of others who were unaware of the inner set.  Also on bleeding the new M/C, I bled initially by cracking the flare nuts at the cylinder first to reduce the bubbles in the system, the result being a "pretty good" pedal before bleeding.

I really appreciate the advice on adjusting the push rod gap, wasn't sure what the correct measurement is.  I'll check that out asap.

I don't believe there is any firewall damage, given there's no discernible movement when pushing hard with no vacuum assistance, but I'll have a look.

As always Bruce, your advice and experience is really appreciated - wish you were up here for us to visit.

Regards

Rob