420R brake bleed problem

[Jaggies Dad]

I built my 420R in 2016 and had no problems bleeding the newly filled brake system at that time although it took repeated bleeds to get the pedal feel right. After my last track day at Knockhill I felt the travel was increasing so I decided to bleed the brakes again and started yesterday. I follow the procedure in the manual nearside rear, offside rear, inboard nipple front nearside, outboard front nipple nearside and repeat offside front. The problem is immediately after bleeding any of the front pistons the pedal goes to the floor and there is no resistance but after leaving it to rest for 20 secs or so resistance returns and we can bleed again. I’m am using the traditional hose under oil with my wife as pedal pusher. We keep bleeding until no bubbles are visible for 3 consecutive bleeds and then move on. However until the pedal achieves resistance immediately following a bleed I’m clear there must still be air lurking around.

We have been bleeding for several hours and if you let the brakes “rest” for 20 secs or so after bleeding any of the fronts they feel normal and I have braking whereas if I bleed either rear brake the pedal resistance feels fine immediately. But any bleed from any of the front nipples immediately results in no pedal resistance and no braking at any wheel until it’s “rested” for the 20 secs. I’m due to drive to the Isle of Man on Monday but dare’nt until I understand what is happening. 

Its as though air is being reintroduced somehow but there are definitely no oil leaks as I have scrutinized every part of the circuits. 

I am suspecting I have a problem with the master cylinder but even if there was a seal problem I can understand how air could be introduced. Needless to say I am ensuring the oil level in the reservoir never drop anywhere near the min level.

Does anyone have any ideas or a cut away drawing of the master cylinder I could study?

Thanks in advance.

[Tazio]

Are you undoing the bleed nipples too much and allowing the air to pass back past the threads?....I know I did ....once....hmmm

 

[ChrisC]

Given you didn't  have a problem with the components before you started bleeding apart from the soft pedal, I assume it's a problem with the bleed. 

Are you still getting air out?

Any local members you know have any pressure bleeders or vacuum bleeders.   I use a bleeder that attaches to my airline, and finish off we a few manual pumps on the rear calipers.  Even that takes a few attempts before all the air has gone.  

[Jim 123]

Some off the wall thoughts.....If your car is 2 years from build, have you done your first brake fluid flush and change. Sometimes a new brake system gets an accumulation of piston crud during bedding in which may make things a bit contaminated around the master cylinder pistons. You might notice deposits in the bottom of the reservoir.  Are you leaving the handbrake off and untouched after you have bled the rears and move on to the front.  During the 20 secs wait with the fronts, can you easily rotate all hubs.

[DJ.]

I suspect it is the master cylinder too. There was no air before your track day and then the pedal didn't feel right. Something must have happened to the system to cause that. If the calliper seals go, you will get brake fluid escaping. I think the master cylinder seal that pumps the front circuit has failed, but I am only an amateur mechanic. You're definitely correct not driving your car.

Duncan

[Jaggies Dad]

Thanks guys all very helpful but here’s an update. I repeated my bleed procedure this morning and no air visible at any wheel but I still require the 20 sec wait after bleeding any of the front before pedal resistance returns. 

I wondered if I was opening the nipples too far so this morning I only opened enough to get flow but as no air was visible I am happy air is not the cause. My fluid was still pristine although there were some very small black specks on the bottom of the reservoir so this may indicate a rubber component failure in the MS but I am talking a very small amount - no more that 6 specks which I am guessing are less that 0.1mm in diameter. As I have flushed well over 750 ml of fluid through the whole system it is effectively all new fluid but the particles are trapped at the bottom of the reservoir.

So, with pedal feel back to as before I decided to take it for a tentative test drive. All appeared as before so I took her out again undertaking several track day like braking on a quiet back road and again, all feels as before. However, this time I measured my disc temperatures with a laser spot temperature gauge I have and it revealed both front around 45C nearside rear 70C and offside rear 58C.

Intuitively this does not seem right as I am sure all cars should have a front brake bias so again this may indicate something amiss on the front brake piston in the MS. 

I have decided to give Caterham a call in the morning and another brake specialist I have come across on Piston Heads, BG Developments, and see if they also conclude I should replace the MS. 

I’ll update this with the conclusion for your edification.

Many thanks again for taking the time to respond.

Best

 

[SM25T]

Rears don't cool as well as the fronts, as little airflow under there.

[ScottR400D]

The brakes are completely different in size too. I wouldn’t think temperatures are that indicative as long as they’re broadly similar across axles. 

 

[Jim 123]

Like others have said the issue tends to point to the master cylinder and inhibition of the replenishment of the piston for the front brake circuit from the reservoir. Is it worth checking the pedal adjuster inside the pedal box in case something is coming loose as this may affect where the resting position of the piston seal sits relative to the m/cylinder feed? Also could it be detritus (black specs) blocking the feed from the m/cylinder?

[Jaggies Dad]

To answer Jim’s earlier question: yes, the handbrake is off throughout and yes, during the 20 s wait period there is no braking at any wheel.

You may all be right regarding disc temps and I’ll check the linkage and mechanism this morning. I’m hoping BG Developments can give me a definitive diagnosis if I’m safe or not. More anon.

[John Vine]

... or a cut away drawing of the master cylinder I could study?

There's a graphic of a generic brake MC here.

JV

[SimonR300]

I had a lot of trouble bleeding my brakes after replacing the two rear brake lines.  I have this M/S

 https://caterhamparts.co.uk/master-cylinder/29-master-cylinder-apracing.html

and race spec AP calipers.  I too pumped litres of fluid through while trying fancy valved bleed nipples and all sorts. In the end, I reverted back to a set of new standard nipples from CC.  No more air coming through but I still had a very soft long pedal.  So I replaced the seals in the M/S

 https://caterhamparts.co.uk/master-cylinder/1817-seal-kit-1316q-ap-master-cylinder.html

and removed the rear calipers so they could be tipped up.  They are mounted horizontally and trapped some air.

The old bleed nipples had been installed by DPR Motorsport with PTFE tape which is a no-no for me in case some ends up back inside the caliper piston.  There was also O-rings on some of them, so clearly they had trouble with air leaking in past the threads.

The m/s seal kit (CP4627) came with an exploded diagram and details of the slight differences between the main 4 seals.

I was so desperate that I also bought a new M/S in case changing the seals didn't work as time was running out before an event.  Then after that event, I bled the brakes again and got a bit more air out.  Nightmare job.  One of the hazards of buying an ex-race team car that has had lots of work done on it and some of the threads (ie caliper) used more than a road going car.

I did get on well with the Eezibleed kit, but you still can't beat the foot on pedal method if done correctly.

 

I should just add that once I had solved the various issues with air in the system, I also had an inconsistent pedal and it was changing the seals that was the final fix.  I certainly had a fine layer of black coating the bottom of the reservoir which I couldn't thoroughly clean out so I got a new reservoir to go with the new seals as the reservoir cap thread was leaking too.

[Dave_P]

Good communication with the pedal pusher required. The pedal should be pushed steady down, while you call ,down, down,down. Make sure to close the nipple  while they are still going down. Then call off. This way air cannot sneak back when the pedal is at full stroke, while nipple is being closed.I find this works 9 out of 10. Occasionally you will get an air lock and I find opening the bleed a good way then a bang on the pedal moves it downstream. Then revert to normal bleed. I have never used an easy bleed so cannot comment on the effectiveness.

Hope this helps you and others      

[aerobod - near CYYC]

Pressure bleeding (0.7 to 1.0 Bar / 10 to 15 PSI range) combined with tapping the caliper with a small nylon mallet during the bleed to loosen any trapped air bubbles, works well for me. I undo the rear caliper sliding bolt nearest the bleed nipple, then rotate the caliper upwards while maintaining about 25% pad contact with the disk to avoid the piston being ejected, by supporting the caliper with a small block of wood (I do have the flexible rear brake lines, though). I have found this method works as well as conventional methods, but is a lot quicker and less error prone.

[SimonR300]

Yeah, I did something similar James with my rear calipers, also flexible hoses.  I rotated the caliper so it was upright, (nipple at the top) hung it on a cable tie and put a small block of MDF about the same thickness as the disk between the pads.  I'll try the tapping with the mallet next time.