Soft Brake Pedal Diagnosis

[Dom Williams]

Good afternoon all,

I am trying to diagnose a soft brake pedal...

For context, I was at Curborough a few months ago and whilst not enamoured with the braking capabilities, the car did stop. Until it didn't. The pedal went to the floor in the afternoon - the MC reservoir had pissed a load of brake fluid over the bulkhead (argh!)...I suspect it was overfilled.

Assuming it was a poor seal between MC and cap, I bought the reservoir cap without float switch (but with "sealing" bellows) to and for good measure did a complete flush of the system. The pedal feels pretty good at first pump, quite hard, however starts to get softer as I keep pumping - to the point that after 10 pumps the pedal is very long and can just about reach the end of its travel (with a big leg drive from me). I would expect it to be like standing on a brick wall whenever I touch the pedal.

So, likely culprits are:

1. Master Cylinder - this was in theory changed by the previous owner a couple of years ago (for a brand new standard MC from Caterham) - I don't know if it was ever fitted (I have the invoice) but it does look like it is fairly new, so I suspect it was. I have read lots of posts suggesting that the more recent MCs are hit and miss and that the seals might have gone.

2. Air in the System - have worked my way through a LOT of RBF660 flushing the system (1.5L now), and done all sorts of things (had the rear callipers off and turned upside down, taps with a mallet, handbrake trick etc etc)... there are definitely no air pockets.

3. Brake Pads - the previous owner put brand new pads on all around, however they are standard ones from Caterham and I have read that the rears can give the pedal a spongey feel.

4. Rear Callipers - I read that if these are binding and not actioning correctly this may cause a soft pedal. They look to be working correctly.

5. Leak in the System - Car is currently on axle stands - I have been all over and cannot find any leak.

6. Such a strong leg that I am elastically deforming the pedal - unlikely, I haven't been to the gym in a while and steel is usually deforms a little more plastically than what I am seeing

What do people think the likely culprit might be?

Thanks in advance.

Yours,

Dom

[John Vine]

I think the prime suspect is #1 "... the seals might have gone."

I had a clutch m/c do much the same thing a few years ago.  I replaced the entire m/c, but I also fitted a seal refurb kit to the old one to give me a spare.

JV

[7 wonders of the world]

how are you bleeding the rears, the calipers need partial removal in order to expell all of the air out of the calipers

Which M/C are you using the Standard version or the AP Race version.

[SamC]

If you hold pressure on the brake pedal and your foot drifts down then it’s a Master Cylinder seal (assuming no visible puddles!)

The fluid on the bulkhead could be from the fluid passing over the seal and being rapidly returned to the reservoir when you take your foot off the pedal. This obviously can’t happen now.

The master cylinder is the only place where the pedal can drift down but you have no external fluid loss.

None of you other options would explain the pedal getting longer each pump (I suspect the pedal may not be returning fully).

[Colin_T]

Second what SamC said 

[Wrightpayne]

If you’re changing the m/c look at the AP uprated version. There are other outlets that sell them other than Caterham. I got mine from BG developments. I believe they were an option on Westfields too.

It does alter the pedal characteristics - shorter travel & more leg force required. I soon got used to it for a mainly road car.

Ian

[Beagler]

I can recommend the uprated Master Cyl and the use of the Sealy Bleeder Unit It means positive pressure as you close the bleed nipple and you can flush and use a good volume of brake fluid.

 

 

Edited November 28, 2023 by Beagler

[Dom Williams]

Hi all,

thanks for your feedback.

Just to respond to some of the questions:

@7 wonders of the world, yes rear callipers came off and got a tap with a mallet before bleeding and during, also tried actuating the handbrake to clear them completely. Currently on a standard MC, in theory new in 2020 from CC

@Beagler, thanks for the recommendation - I use a Gunson Eezibleed attached to a tyre at circa 20psi to keep the system pressurised. I also had my father on nipple-opening duty and fluid collection whilst I (gently) pumped the brake pedal to push more fluid through in the vain hope it was air in the system.

@SamC, thank you for your suggestion. I will try that when I get home. I like your answer as it is the suggestion I have heard that explains both Curborough and bleeding woes.

Re: an upgraded/race master cylinder...it seems that CC no longer sell them (currently out of stock) and AP no longer make them. They are also advertised at £417.77 + shipping, which is stupid. 

One of these is still a 13/16 bore (same as race MC), is still made by AP, is bulkhead mounted, and is less than £200:

https://apracing.com/race-car/master-cylinders/flange-mounted-types/vertical-flange-types/cp2623-type

It looks like the attachment of the reservoir and copper tube(s) would need to change with respect to standard but doesn't look impossible unless I am missing something. Has anyone tried something similar?

Thanks in advance

[Richard Price]

Quote

One of these is still a 13/16 bore (same as race MC), is still made by AP, is bulkhead mounted, and is less than £200:

https://apracing.com/race-car/master-cylinders/flange-mounted-types/vertical-flange-types/cp2623-type

It looks like the attachment of the reservoir and copper tube(s) would need to change with respect to standard but doesn't look impossible unless I am missing something. Has anyone tried something similar?

Dom,

'not sure of the legality of fitting a single one of those? The original (and the AP upgrade) are dual circuit, so, theoretically, should one half of the system suffer a failure, you'll still have some brakes available from the other half.

It may be worth putting a "wanted" add to see if anyone has a standard M/C lying redundant having upgraded to the AP M/C.

I modified my own car (about 15 years ago) to use dual master cylinders and an adjustable bias bar. I use a couple of fairly cheap Wilwood master cylinders with integrated reservoirs. It's initially a lot of work but makes optimizing front to rear brake balance easy, and avoids messing around with different pads front and rear to try to achieve reasonable balance.  Since modifying my own car, I've done another couple of very successful hillclimb cars.  

Edited November 28, 2023 by Richard Price

[JP]

I've just replaced my MC as part of my rebuild.  I went for the uprated, which I had before.  As part of the hunt for a better deal I spoke to a number of other retailers. I was told that AP made both MCs for Caterham (std and uprated) and only Caterham and Redline are authorised to sell them so I just had to take the £400 cost.  In case it is of use to anyone, I took a pic of the box with the spec of the uprated MC:

[PaulCo]

Hi Dom,

I too copied the Richard Price twin master cylinder system, and can vouch for the effectiveness and ease of adjustment. As a result, I have a standard (as fitted to the 2003 Superlight) master cylinder sitting in the garage, so if you wanted a simple replacement, it could be available at a reasonable price.

Paul

 

[Dom Williams]

@Richard Price thank you for the feedback - I had not considered the legality side of fitting the single-circuit MC. I will look into creating a dual-MC setup - I would be interested to know which what sized bore on the MC and which pads you went for. I imagine I will see you next year, assuming you will still be competing(?)

 

@JP thank you for the PN, I will see whether I can find one at a sensible price or whether I have to bite the bullet...

 

@PaulCo thanks for your kind offer; I think I would prefer to take this "opportunity" to either join you on the twin MC or an uprated MC.

[Richard Price]

Dom,

You should have blatmail.

[p.mole1]

One thing to check is the rear pads, from memory  I think there are two raised "prongs" for want of a better word. These must fit into the corresponding cut outs in the caliper piston. There are 4 cut outs. I made this mistake when I wound back the pistons to fit new pads and didn't align the pads with the cut outs, this made the pedal soft and I used quite a bit of brake fluid before I realised my mistake