I recommend getting hold of a vacuum bleeder. After struggling using all the above suggestions, I borrowed one and got an instant result. My personal theory is that all the pressure systems compress the air bubbles and they stick in the nooks and crannies. Vacuum bleeders drag the air out with the fluid.
Never had any problem with my system, takes about 1 h to put the 7 on axle stands drain the system refill and bleed single handed.
If it is so bad the only answer that seems logical is that the master cylinder is letting fluid past the main seal but the fluid is being retained by the secondary seal so no leak is showing. I would be stripping the master cylinder if it were mine, if that proves a red herring then a small fire may fix the issue!
R.I.P. SLR No.27.
You wouldn't get any leaks if the seals were worn and letting by!
You just are unable to maintain pressure
I would second the vacuum bleeder though its quote possible for them to pull air in around the bleed nipple thread, instead of drawing out the fluid, and give the false impression that there is a lot of air in the system.
you either need nipples with loctite on them as used by most OEM's now or use ptfe tape on your nipples.
I knew there was a reason why I did it the old fashioned way*rofl*
But if you use a Loctite type product on your brake nipples I'd use a thread sealer with a low breakaway torque rather than a conventional Loctite thread locker.
Sorry had some IT issues and not been able to post for 24 hours.
To answer an earlier question I am using Mobil 1 DOT 4 fluid and it is all new unused and purchased very recently so I hope the quality of the fluid is not an issue.
I am not ultra impressed with the bleed nipples on the front calipers so perhaps some sealer on the thread may help but will that make any difference if there is no leak once they have been done up.
I don't have a vacuum bleeder but I will ask at work - we have a few useful brands in our portfolio - and if I can borrow one I will. We have a nice digital pressure system but it's very expensive and we don't have a demo one to hand where I am that I can borrow.
I plan to talk to Redline about new seals for the master cylinder. Looking at the Caterham web site I think I have the standard one not the unrated AP one but it's not clear and my car was built in 93 so parts have changed a little since then.
If I do get new seals how easy is it to replace them? Based in my performance so far it needs to be simple.
The Uprated MC has a red cable tie around it, of course that may of been cut off. There is an identification code on the MC body as well. The early Uprated MC reservoir mount was different from standard. If you decide your MC is at fault my advice would be buy the Uprated one seeing as you have the Uprated calipers. Redline don't or didn't when I asked (2016) provide the repair kit for the Uprated version, that is sourced via a motorsport team in North Wales of which the name I can't quite remember!