Standard Master Cylinder Degradation

[Dom Williams]

Good afternoon all,

after having had a terrible brake pedal last year, I have finally found time to revamp the brake system and look for the root cause. Everything looks sweet (despite the car being 28 years old now)...the only thing that doesn't is the standard AP Racing M/C. This component was replaced by the previous owner in 2020 (I have the receipt from Caterham Parts); I believe the car to have done circa 800 miles since then (796 of which were on the road and did not require much braking at all).

I could see some rust at the entrance to the chamber (hidden underneath the rubber boot), but decided that was unlikely to have an impact on sealing/performance due to its location "outside" of the sealed volume. I decided to get the inspection department at work to pop it on the CMM and measure the bore diameter along the length of the M/C so I can see if it is outside of tolerance; the results are attached for anyone who is interested. 63 microns between (measured) largest and smallest diameters; I suspect that it is absolutely fine...

AP Racing have refused to give me a tolerance on the bore diameter as the know-how belongs to Caterham and it would breach their contract (I fully understand and respect this); tomorrow I will call Caterham and ask them for an engineering drawing/a nominal diameter + tolerance and see if they can help...alternatively if I send them the results they can tell me if it is within tolerance (but I won't hold my breath) 🤞

I cannot see any "wear" on the seals/piston, although I must admit I do not know what a worn M/C seal would look like.

I then asked the inspection department run a boroscope into the M/C to get some photos; it looks utterly horrid... 🤮 (photos attached).

Has anyone else had this happen to their M/C (particularly if purchased in 2020!)? I can only imagine it was stored in a humid, damp environment (with no plastic plugs to seal it) before being fitted to the car...whether that was Caterham themselves or the previous owner who can say.

Hopefully I can get some useful feedback that I can use to make a decision on where to go/what to do next!

Thanks in advance.

Yours,

 

Dom

220403 Master Cylinder Measurements.pdf

Edited May 6 by Dom Williams

[Gridgway]

Presumably the solution is to just replace the M/C?

How often has the brake fluid been changed since 2020?

[Dom Williams]

If the rust is merely superficial and the bore is within tolerance, perhaps new seals are sufficient(?)

Should it need new seals after just 4 years and so few miles?

A new M/C might solve the issue, however it's not exactly a cheap consumable...so rather than spending proper money on a new one, I would like to first ascertain that it is the root cause.

I have bled the brakes a few times in my ownership (doing usual tricks of pulling the handbrake for the rears, removing the rears and turning upside down and tapping with a rubber mallet, trying to dislodge any other air bubbles in the system with a mallet, with someone operating the brake pedal, also using a pressurised Easibleed system, together with someone who has owned Caterhams for years and has never had issues with bleeding their brakes...) - I don't think that's the issue. 

[Nigel B]

In 2019 I part exchanged the live axle Seven I’d owned from new for an eighteen month old 420R.

On the test drive at Caterham Gatwick I noticed that the brake pedal had far longer travel than my live axle car, with an uninspiring ‘mushy’ feel. I presumed the brakes just needed a good bleed, as that was how it felt to me.

However, I replaced the fluid, using a Gunson Easybleed & my usual technique & the pedal felt exactly the same. I assumed I must have air in the system & I did a lot of research on Caterham-specific bleeding techniques & tips, but nothing I tried made any difference. I tried gravity bleeding, reverse bleeding, different brake fluids & all the methods you mentioned above, but the pedal always felt mushy & vague.

In 2021 I had a set of throttle bodies fitted by Luke Stevens & the following month, I had the car set up by DPR. I asked them both for their opinion on my brake pedal feel & they both said the feel was good! DPR offered to fit a ‘high effort’ race spec brake pedal to reduce the travel & bingo! The short travel & positive feel I remembered from my live axle Seven was restored.

I may be wrong, but perhaps that’s the answer?

[Guy Lowe]

I have a 30 year old S3 with the original AP master cylinder (new seals 10 years ago) and my pedal is great, only 1/2”  travel, solid as a rock and perfect for heeling & toeing. People who have driven my car often comment on how good the brakes are for standard set up. 

[Dom Williams]

@Nigel B thanks for your comments - it sounds like the new standard master cylinders might not give quite the same pedal feel as those fitted to our cars 20+ years ago...I would like a "Race M/C" (13/16" bore) but they are £440ish new...which is a lot! I have a couple of options lined up for a second-hand one, I suspect it makes sense to just go down this route and put this to bed.

@Guy Lowe thanks for your input - my father used to have a standard M/C in his Sevens (1996 and 2001) before he upgraded to the Race M/C, but he also agrees that the pedal feel I am currently getting does not match what he used to have.

[SamC]

@Dom Williams, Firstly, Love the filename that comes up when you open the CMM file.  Glad it isn't just me that gets grief when I get 'homers' done at work! 😆

Secondly...  From experience, the corrosion will have put pits in the surface.  This will just give the fluid somewhere to wipe past the seal, having a more negative effect than the bore simply being oversize.  Typically a hydraulic sealing bore of that size (0.7"/17.78mm) will have a maximum tolerance of 50microns ish, so yours seems on the upper end but not outlandish.  You could try getting it honed out to clear up the corrosion and see how that ends up?  

There is a company called Hone-All in Leighton Buzzard that are good - may be worth a shout.

Edited May 7 by SamC

[Dom Williams]

@SamC I definitely renamed the file to hide that 😄 Clearly the old filename is still a parameter somewhere in the PDF!

Thank you for the info re: typical tolerance, I will have a think about the honing - you have me now wondering whether I can get it bored and honed to 13/16" and buy the Race M/C piston to fit 🤣 (sure that's some useful weight saving too )

[Gridgway]

I'd go along with the idea about which pedal you have - high or low ratio/effort.  I have had the k-series era big brakes which had a terrible feel until I changed to the high ratio pedal which was like a magic switch.  I'm currently bedding in the brakes on my recommissioned 80s 7 which are pretty mushy.  I need to work out which pedal I've got.

For your master cylinder, if the corrosion was causing an issue would that not create a leak with fluid coming past the seals?  I'd try removing the corrosion with some gentle scotch-brighting as the first port of call then try it with new seals.

My money is that the squidgy brake feel is caused by a low ratio pedal.

[Dom Williams]

@Gridgway thanks for the idea re: pedal type.

The pedal gets soft on the second compression (when stationary), it doesn’t go quite to the floor but almost. I would guess that the fluid is going past the seal between front and rear circuits - so it is not leaking out, but rather leaking from one chamber to another.

Just for some context, my father (mentioned above in one of my previous posts) actually owned this exact Seven (although there were a few owners between him and me). Unless one of said owners changed the pedal type (I have what I think are all of the invoices and there’s nothing), the pedal type will be exactly what he had (with no issues). I will regardless check the pedal to make sure. Thanks!