Brake bleeding tips

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Nigel B
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Brake bleeding tips

Sorry about the rather long-winded post, but following on from my thread about pedal box adjustment & master cylinder pushrod thread, I’d really appreciate any advice people have on brake bleeding.

I’ve had my 420R since Easter 2019 & I’ve never felt very happy with the brake pedal feel. The pedal has always had a somewhat mushy feel, as if there’s air in the system. The brakes do work & the car stops well, but the mushy pedal doesn’t inspire confidence & doesn’t provide a stable platform for heel & toe. The car has the standard two pot AP front callipers with the Sierra type rear callipers & the uprated AP master cylinder. 

I’ve bled the brakes three or four times, with newly purchased, decent quality fluid & while that has given me a firmer pedal, I still feel it could be improved. 

I had a live axle 7 before I bought the 420 (standard Girling callipers at the front, Ital drums at the rear & the standard master cylinder) & with that car I always found brake bleeding at the start of the blatting season a very satisfying job, resulting is a firm, confidence inspiring pedal.

I’ve tried the traditional, two person method, pressure bleeding using a Gunson Easybleed, vacuum bleeding & (in desperation!) a combination of all three methods at the same time.

The Sierra rear callipers seem to have a reputation for being awkward to bleed properly, so I’ve paid particular attention to those. I was told an effective approach was to remove the brake pads, wind the piston in fully and then bleed them, as that helps to push the air out of them. I’ve also disconnected the handbrake cable from the callipers, wound back the pistons & pressed the brake pedal firmly, to ensure the callipers are correctly self adjusted, before re-connecting & adjusting the handbrake. I’ve also tapped the callipers with a rubber mallet to attempt to dislodge air bubbles & vigorously waggled the handbrake lever during bleeding.

Another favoured method seems to be holding the rear callipers up high & ensuring the bleed nipple is right at the top when bleeding. I have the standard solid brake lines at the rear, so it’s difficult to move the callipers much. I have bought a set of flexible braided hoses, that I plan to fit before my next attempt at bleeding, so hopefully that will improve matters. 

It really does feel to me as though there’s air in the system somewhere, although I’ve pushed many litres of fluid through & I’ve got to the point where I can’t see even the tiniest air bubbles emerging. I can’t see any leaks anywhere, the fluid level doesn’t go down in the reservoir. If you press & maintain constant firm pressure on the pedal, it doesn’t sink any further, beyond the initial mush. 

I have felt tempted to try the ‘high effort’ pedal, but I feel I’d rather have another try at optimising what I have before I go down that route. 

Have I missed anything? Or is it simply a question of bleed & bleed again, until the air is driven out? Any tips would be much appreciated!

Tazio
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I found that after fitting the Race Master Cylinder it gives a much firmer pedal, than the standard fitment.

Nigel B
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Interesting. Is that a master cylinder designed specifically for race use? I just had a look on Caterham's website, but couldn't see anything. I already have the 'uprated' larger bore AP master cylinder that lots of people seem to find is the answer to mushy pedal problems.

7 wonders of th...
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Are the rear pads located corrected wit the pip in one of the 4 slots in the pistons.

Are the front wheel bearing correctly adjusted - excess play will allow the pads to be knocked back during driving therefore the first application resets the pad position.

Check on of the the pads hasn't suffered partial delamination where the friction material cracks or starts to come away from the back plate

To be honest gravity is one of the best methods, as the fluid only goes one way - you can inadvertently draw air into the system via the nipples threads if you using the 2 man method.

You might have a master cylinder that hasn't been shimmed correctly though AP are usually pretty hot on getting this correct

You might remove the anti squeal shims, especially if they re the wolverine type too.

 

 

 

Nigel B
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Yes, I was careful to ensure the rear pistons were in the correct orientation to locate with the pips.

I must confess, I haven’t looked at the front wheel bearings, although, as the pedal feels mushy when pressing the pedal with the car sitting in the garage, I wouldn’t imagine it’s pad knock back that’s causing the issue. I once experienced really bad pad knock back on a Triumph TR4A I had, so I think I’m fairly well attuned to how that feels. Having said that, I’ll still have a look at them...

The rear pads look okay, though I’ve never taken the front pads out, so I’ll have a look at them too. I’m not sure if anti-squeal shims are fitted. I think a thread in the archives concerning pedal feel mentioned shims as a possible issue.

I’ll definitely give the gravity bleeding method a go too. It’s not something I’ve ever tried before. I certainly have the time to try that at the moment & I have a five litre bottle of brake fluid sitting in the garage!

Thanks for the hints & tips. They're all very welcome! Thumb Up

paul richards
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It sounds like you have led the brakes correctly. It may not be a bleeding problem. I have experienced a longer pedal when rear pads have a lot of movement before contacting disc. Make sure pads are moving freely and properly seated on the piston and that they move freely with "butterfly" wires not pulling them back too far.

Paul Richards

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"It's only a car club"

ScottR400D
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Nigel, if you get an answer, please let me know!! My set up’s the same as yours and I’ve had exactly the same experience. 

Having had race cars with similar, simple setups, albeit with two small MCs instead of the large one, and having pedals that were rock solid, the 7’s slight sponginess was a surprise to me. 

As suggested by 7W, I’ve always felt it was a mechanical issue but I’ve no idea what.  

Nigel B
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When taking the rear callipers apart, I was careful to clean them up with brake cleaner & grease the relevant areas. I got a tube of Mintex grease for the purpose. I haven't looked at the front callipers though, so I should check they're working as they should. I've never heard of 'butterfly wires'! Confused

Peter, it makes me feel better that you're experiencing the same problem! I'm glad it's not just me. I did start to wonder if it was my imagination, or if my expectations were simply too high, but then, when I sit back in the car & prod the brakes, I immediately think 'No, it should feel better than this!'. It's frustrating that an ostensibly simple & routine servicing task is proving to be so problematic.

My experience with my live axle 7 was similar to your race cars, it had a really solid pedal with a feel that immediately inspired confidence. I had always assumed that all Caterhams had fantastic feeling brakes. 

Geoff Brown
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Not sure what all you chaps are doing wrong as it must be a fundamental oversight &/or a combination of tiny things? Of late it seems to be the norm & not the exception to have a poor bleed & pedal feel.

Three Sevens owned, use of the Gunson Eazy Bleed - no problems.

During my R400SD LSD job I overhauled the rear calipers & changed the pads. Eazy Bleed & one litre of DOT 4 later no air & a good pedal. 

Forgive me if it may appear that I am being a bit patronising but I am not. Just cannot get my head around what could be the problem with such a simple hydraulic system.

 

 

 

 

ScottR400D
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Geoff, my car was built almost 6 years ago, so it’s not just a recent thing. 

Over the years I have bled the brakes of countless cars. I’ve done the seven 3/4 times since I built it. I have no doubt I’ve done it properly, or that there’s no air in there. I think the bleeding is a bit of a red herring. 

The thing is, it’s not a BAD pedal as such,  just not what I would have expected from such a basic system or what I’ve experienced in race cars. My son, who used to race those cars agrees that the pedal’s not as solid as it could be but doesn’t think it’s bad. Most people wouldn’t notice. 

I’m not sure that pad knock back etc is an issue either, the slight give is always there even once the brakes are fully on.

 

 

David_Long
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My 2p . . .

Gravity bleed them - i.e. let them drip for 5-10 minute each corner in turn and do that twice. Wheel bearings can cause a 'springyness' even when stationary as the discs tend to move with the weight of the car. Pads themselves could be changed to see if that improves things. Particularly the Sierra rear pads as then can go very soft if overheated. Check there are no damaged hoses and replace them if they are old. Handbrake - try bleeding with the handbrake on, and check the feel with the handbrake applied.