Bleeding brakes

[Morris]

Any chaps on here had any trouble bleeding the brakes on you XI (standard Midget drums at back and 'new' Midget calipers at front)?

All the air is out of the system, but the brake pedal is springy (not spongy) for the first third of its travel before firming up nicely. I've adjusted the rear shoes to be against the drums but this doesn't improve the situation.

It's been suggested that the master cylinder may be lower than the calipers resulting in the fluid flowing away from the calipers upon releasing the pedal.

If this is the case then a residual pressure valve in the brake line could help.

I haven't seen any XI postings relating to this implying that it could be something else.

My front end may currently be a little high given that I've set the ride height at the front but not yet at the back of the car.

Has anyone else experienced this?

Many thanks.

Morris.

[Splat]

Hi Morris,

I first saw this just after you posted it. And I’ve almost replied a couple of times. But didn’t

And I now see that it’s been viewed 23 times, still with no reply.

The best that I can offer is…. I haven’t a clue!! I think that it’s one of those things where…. you actually need to be there and see it/experience it for yourself. However, I don’t think that it’s a low-lying master cylinder thing. That just doesn’t sound right. More likely, it’s actually a problem with the master cylinder itself. What are you using? I had (maybe similar sounding, in hindsight) problems with the AP dual circuit racing master cylinder myself, a few years ago. Turned out to be a leaking cylinder, cured with a new unit.

[Morris]

Thank you, yes I do tend to agree that it's unlikely to be the relative heights if the MC and calipers. To prove this out I jacked up the rear of the car ridiculously high but with no improvement.

The fact that no one else has experienced this must also be a clue to something else being the cause!

As to the master cylinder type, the Westfield pick list describes it as a 'large bore' tandem master cylinder. Hopefully the actual bore size will be marked on the casting so I'll take a look.

Are defective units quite common?

(My old Marlin Roadster used a tandem MC from a Morris Ital that I picked up from a scrap yard, replaced all the O-rings and it worked a dream...).

[Splat]

I think that’s probably the same unit as mine. It’s an AP racing unit, and they’re not cheap! I ended up getting one from Caterham Cars, as they were both the cheapest and one of the very few suppliers with stock when I needed a replacement.

Like you, I just couldn’t bleed the brakes to give me a satisfactory pedal feel. They were always spongy (from memory, as the failure must have been eight or nine years ago). Replacing the master cylinder, like for like, gave an immediate, positive result. I run standard drums on the back, but drilled 8” rotors and MGB callipers with Mintex pads on the front, and the level of retardation is more than adequate (although you have to remember that there’s no assistance and the pedal needs a good shove!).

Good luck!

[jonclancy]

Hi Folks,

Sorry - I’m late to the party!

My brakes are similar, though I have yet to do a fluid change for this year. Passed the MoT Test fine, but I recall a time when they were really good. Probably about the time I first went to M1144 pads. Really good initial bite and a good pedal feel.

You’ve done the first thing to check (adjust the rear shoes). Are you sure all the air is expelled? The broomstick left overnight on a depressed pedal can help. But…

I read somewhere that what can be a problem is using the old “Uuuuuup…. Down” method of bleeding. The piston travel in the cylinder is much less than full deflection when braking. I assume you are using a new MC unit, but if not, perhaps a small ridge has developed in the bore. This ridge can damage the piston seal when the seal is pushed past during bleeding operations.

Of course, this could all be urban legend and general tosh! However, using a decent pressure bleeder is a better method (I didn’t get on too well with my MityVac, but have a positive pressure set up to use now).

I do seem to recall that there was some information about the MC bore size versus slave cylinders. I’ll see if I can find it. I have a bog-standard single cylinder set up on my car.

Cheers

Jon

[jonclancy]

https://www.westfield-eleven.org/forum/ ... e&start=10

https://www.westfield-eleven.org/forum/ ... bore#p2107

For starters. Took a trip down the forum archives back to 2011. This place is a trove!

[Morris]

Yes, thank you for those links. It's surprising what can be found in the archives.

I understand the 'don't press the pedal to the floor whilst bleeding' if the M/C is old as the seals may get damaged on formed ridges. Mine is a new M/C so it should be good along the whole travel.

As for it being 'AP', the ref number on the tag (L17B ??BHGL033A) doesn't align with AP part numbers and there is no coloured band on it indicating the bore size so could it be an 'AP style' M/C or perhaps that's the cynic in me speaking!

I'm sure (as I can be) that the air is out of the system, and I've tapped the calipers with a mallet for good measure.

At least I now know that the bore is 20 mm, but £300+ for a new one.......? Would be nice to have remote reservoirs to save digging down past numerous pipes/ cables to reach it in its current position.

[Morris]

[Splat]

Definitely AP. Here’s the box, complete with part number! (I keep a small pile of postage-friendly boxes. Just in case!)

[Morris]

Ah ha thank you, I don't think that specific one is sold now but the equivalent may be this one as fitted to Caterhams (if I can upload a screen shot). Being a cheapskate, £365 is a bit on the steep side though for me.

I learnt today that AP Racing and the manufacturer of AP units as provided as standard with the kit are not connected and so may be poles apart in quality.

Having said that I may now have the opportunity to buy a new original kit supplied unit from a fellow builder who has upgraded his system, so I may now be ok fingers crossed.