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997.2 C4S Brake Bleeding issue


25 Mar 2016
After a bit more advice on a rather odd issue.

Car in for a service, as I understand things the service is completed including the brake fluid change completed without issue (assuming I have the sequence of work correct).

As expected the humungus wish list of repairs appears, some discussion and changing both front solid lines from flexi-pipe to caliper and one rear solid pipe from flexi to caliper are authorised due to corrosion.

Now at this point after the lines are replaced there are difficulties bleeding the system, the pedal after bleeding isn't satisfactory, some more work and the pedal is deemed acceptable.

Vehicle is left overnight and the following morning the brake pedal as it is pushed down there is some resistance, continue applying pressure and the pedal continues to lower without resistance before eventually feeling solid and applying the brakes. The pedal by now was a couple inches away from the carpet.

Any thoughts on why the brake pedal went from acceptable to nearly nothing by sitting overnight? System was bled again and back to acceptable.

The question is just about trying to understand what happened and could it happen again if further work on the brakes is undertaken.

Any thoughts on the why appreciated.
These problems are usually air in the system.

Bleeding the brakes ONLY improves things if there is air in the system.

Working on the brake pipe system is going to introduce air.

That's it really.
I agree with Magic919 .. air in the system .. but i'll try to put a bit more meat on the bones ..

If you fast pump a brake pedal with air in the system you will get a hard pedal .. if left then applied it will travel too far .

On the service its possible they did a brake bleed from the outer nipples only when doing the service .. I have no problem with this after doing some testing which if anyone wants to question i will answer with what i found but i have already posted this in the past .

When they changed the brake lines then air is in the brake system .. All nipples have to be bled from to remove the air .. it's possible one was seized ..

You can try and bleed from just 1 nipple on a caliper but usually there will be some air in the system .. embarrassed garage didn't want to say a nipple was seized so had to fix that and then do a proper bleed .

We are not bad people as such but can get caught out by silly things ... brake bleed done , pointed out brake lines but didn't think to check the inner nipple would come undone .. it happens ..

My next thought .. brake master cylinder ... as with any car the bottom part of the master cylinder never gets used .. it does on a brake bleed if doing it manually ... a failed seal on the master cylinder will cause a longer travel after a manual bleed ... in essence the seal it getting either torn on internal corrosion on the bottom part or it kinks .. That tends to be a permanent issue though with a new master cyl required .

Modern cars are also plastic i believe and as such this can't happen .. either way i'm throwing it out there for anyone reading this in the future .

This though is normally on far older cars than yours but it's a thought from me ..

If you have a normal brake pedal .. it should actually be a slight bit firmer than before then its all ok .
Thats great information, so when it comes to the regular 2 year brake fluid change is it worth doing all the nipples ( inner & outer ) or just the outer ones?
deMort, Magic919,

Thank you for your thoughts on this, bleeding was via machine rather than manual process. I was not aware there are 2 bleed nipples on the caliper.

A total of 3 solid lines were changed, all caliper to flexi so I wonder if removing 3 lines (2 front, 1 rear) at the same time, replacing and subsequently bleeding could be an issue.

My real concern is having confidence that the system is working properly and isn't going to be a future maintenance PITA.

It was a scary experience!
Honest answer .. a garage on a minor service will not be removing the wheels ..

Major service and we do and i expect every Indy will so yes at that point we do both nipples .

OPC dont on any service .. at least they didn't when i was there ..

As such it's a bit difficult to undo both bleed nipples on a minor .. you can but it's difficult .. hence a lot of garages will undo the outer nipples only .

years ago i had to change a racing brake fluid to standard .. this is a blue fluid which i was replacing with an almost clear fluid ..

So i measured the amount of fluid coming out until it turned from blue to clear .. i didn't drain the reservoir .. which with the filter that's a bit tricky to remove and as such no mechanic ever will on a brake fluid change i was able to test the amount removed from each corner based on what we usually do .

Starting with N/S/R .. that's the furthest away from the master cylinder ..

I measured 0.5 litres before it went clear .. i undid the inner nipple and it was clear .. no blue colour ..

I then did approx 0.5 ltr per caliper with NO blue coming out from the inner nipple on each one .

Ok so a standard brake bleed for a mechanic .. we do 0.25 per caliper which equals 1 ltr in total .. the correct way is actually 1.25 litres in total ... approx .. you do get slightly less on the other calipers before they are colour changed .

These days i do far more from the N/S/R caliper and slightly less on the others .. still a bit over 1 ltr but i know i've got pretty much as much of the old fluid out as i can .

To be fair .. even if changing 90% of the fluid we are still getting rid on 90 % of the potential water ingress so its negligible .

There will be a tiny amount left in the abs system .. again it's not something we operate for such a small amount and there is a limit of how long we can spend doing something that's of no real benefit .

That in a nutshell is how mechanics work and 90 % is perfectly acceptable if not done as above so i don't have an issue with that .

What needs to be understood .. a negligible amount of water will enter the system in 2 years .. removing 90% of this means there is just about no moisture left in the system ..

Water under high heat conditions will boil .. that causes air which will mean the first depress of a brake pedal will go low .. we are into track day issues here and not normal road usage ..

I have personally had brake failure on a car with well over 20 year old brake fluid .. first press and the pedal went to the floor .. second application and it was fine ... very hot day shall we say on a long journey .. 20 year old fluid and that was the result ..

90% of fluid changed after 2 years ??? there is a limit to what's actually needed imho !! considering 20 year old fluid still works .. just not that well when hot !

The answer to the question of both nipples if you didn't read above ..

IMHO .. No .

Posted this before though .

Thank you again for sharing your knowledge, the conclusion I have is the failure to successfully bleed the system after the pipe change is probably down to only one bleed nipple being used rather than both.

As there were multiple test drives trying to sort this mess out plus my effort after collection the brake pedal will have travelled much further than usual and the seals in the master cylinder are probably damaged.

Your help is very much appreciated.
I doubt the master cylinder seals are damaged as it will be a plastic master cyl and i'm assuming you have a normal brake pedal now ?

If it's back to normal .. it should be slightly firmer to be fair then all is ok .
Well it has given me pause for thought regarding the master cylinder.

Car was thrown back at the garage on a trailer so I can't check the master cylinder material but I did look at replacement parts which were all alloy rather than plastic.
Yup .. i was wrong .. i was thinking clutch master cylinder which is plastic .. brake is ally ...

it sounds like the brake system is still no good though hence they have the car back .. in that sense and from what you have said .. i now think the master cylinder is possible ..

Hard to comment as i don't know what they have done but it is the most logical answer atm .

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