Hi all... not been on here for a while... but thought I would share some valuable insight for you into a critical component of the Aero S1. We ran into this little ditty when undertaking a maintenance overhaul of my dad's Aero over the last few weeks.

The problem first appeared on the overhaul of the brakes... strip and clean down callipers, new pads and seals where necessary and full bleed through. Upon bleeding the brakes, my dad decided to use a airline driven vacuum bleeder... all good on the fronts... but disaster on the rear's. No fluid pulled through.... when using the pedal, one pump and then no pedal, no brakes and no more fluid coming through.

We've seen this before occasionally on other cars when the master cylinder seals go - time to find a rebuild kit.... this is where the journey gets interesting.

Aero S1's brake master cylinder runs directly underneath the exhaust manifold (like a rizla paper away) from the shoe shaped take-off manifold bolted to the top of the master cylinder... and in order to accommodate the reservoir take off's it needed to have horizontal take off ports... seen here....

[Linked Image]

btw - this is about the only image, if not THE only image you'll find of this brake master cylinder on the internet.

The part number stamped into the housing is CP3627-101C.... seemingly an AP Racing part number though you'll find no reference for it in AP Racing's back catalogue of paperwork nor in any of their archive papers available on the internet.

.... things get even more special... given that the only relevant notice you will find relates to the colour of plastic tie loosely circling the master cylinder body... which denotes the size of the thrust rod..... and the notice is that AP no longer make replacement seals for this size.

Furthermore... it highlights that if you have one of these colour tie items you will need to purchase a replacement master cylinder... for which no equivalent part is available nor any that looks anything like the CP3627-101C.

So... what is it???

It turns out... when British Motor Industry disappeared, Lockheed was absorbed into a company called Shift Tech.. who subsequently stopped making most if not all Lockheed products. AP Racing supplied Lockheed products as a distributor / middle man but never produced any..... Upon contacting AP Racing, they knew nothing about the CP3627-101C, they have no service drawings nor design drawings for it.

OK so what is this part, how does it work and what's wrong with the one we have (assuming it is a fault).

More internet sleuthing - it appears that this part was designed and manufactured at the end of the 1990's when lightweight racing cars were packing more punch and needed better stopping power (sounding familiar).... and also needed to manage potential lock up of the rear brakes under hard braking to avoid spinning... think Marcos Mantis 4.6 V8, V8 Westfields etc. who both seem to share this part in sparce numbers.

The Morgan Factory didn't have much info about this part other than saying that bleeding the brakes when they were assembling the S1's was a total pain in the posterior with brake bleeding often taking more than one day to achieve a pedal. [great].. but it seems that the stories from back then were matching what we were finding.

What is also interesting... is that the brake circuits coming off the master cylinder are marked P and S.... P (Primary as it turns out) connect to front brake and S (Secondary) are connected to the rear brake circuit..... now this is where it gets really clever.

What makes this Master Cylinder special and what's going on with it.. why won't ours bleed through..... after much more sleuthing through specialist car forums.... I found out the following:


When the brake pedal is pressed the thrust rod pushes on the Primary circuit compressing the fluid operating the front brakes... .in turn there is a very small bleed by valve which upon primary circuit fluid compression pushes a sleeve of some sort revealing the rear (secondary) brake circuit outlet port and allowing a delay in the operation of the rear brake circuit. There is also a built in bias which is an unusual ratio... 75/25 rather than the usual 70/30 (from recollection).

This delay mechanism is what causes the brake bleeder not to work and why bleeding the rear brakes on an S1 is so hard... there is a small time delay on the sleeve movement and subsequent re-charge of the fluid.

If you use a vacuum bleeder... the sleeve obviously doesn't move as the Primary circuit is not pressurised and the bleed by value does not move the sleeve... thereby keeping the rear circuit closed. So the bleeder is just sucking against a closed port.

From my own experience - if you push hard on S1 brakes you'll lock the fronts and the rear's don't come on at exactly the same time. This was apparently developed to stop these car's spinning from hard braking and the back end overtaking the front... with this design.. the rear will always slightly trail keeping the car the right way round in a panic / emergency push.

Now... what does this mean... if you are bleeding the brakes... the tip from AP Racing was to bleed diagonal brakes simultaneously ... Front Left with Rear Right and then reverse... as we'd already done the front's ... we just used patience... using the pedal assistant in the cab doing the one press at a time method... you get fluid through (one push only at a time)... then wait for the master cylinder to re-set itself... (5-10 mins mind you).. then go again.. one push.

It does work and we checked that all the brakes were good and there was a good pedal.

Now ... this is all well and good... and we don't have a knackered master cylinder.. however .. the proximity to the exhaust isn't exactly going to help... so what maintenance options did I find... and boy this took some finding.

The only eBay one we saw - was snapped up immediately - apparently these are much sought after by westfield, caterham, racers for their qualities and I suspect by people needing replacement parts for cars...

This Dutch company has refurbishment service though this is 250 Euro and sending off to Holland.
https://ccparts.nl/en/product/master-cyl-s-ap-racing-cp3627-2xoutl-m10-1-ab/

In the depths of one of the forums an individual had sent his to BG Developments who seem to be a company that refurbishes old AP/Girling/Lockheed brake parts... the CP3627 is not listed but he had good service from them. I suspect they have all the old sized seals needed for the trust rod.
https://www.bgdevelopments.co.uk


Hopefully this is a useful nugget of information ....


btw - I'm off racing again... I'm launching a YouTube channel this time rather than take pictures and type a blog... if you want to follow us from the start - find us at https://www.youtube.com/@AspectRacing and our channel is called From Garage to Grid. 6 episodes in production and I suspect we'll have nearly 20 across the full season.