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MG MGB Technical - 4 pot calipers

master cylinderrakes with Austin Princess 4 pot brake clippers and Peugeot 505 cross drilled discs. Rest is standard for 66 B, no servo and single brake line. But I am not happy how brake pedal feels. It has rather longer travel and feels a bit soft. To get good brake I have to press it twice or push to the metal.
I thought that is maybe air trapped in lines so I bleed stand way (pumping pedal) then I used pressurized system from top to bottom (brake cylinder
Toni Kavcic

Toni first thing to do is clamp of both front flexible hoses, then try your pedal, if you get a good pedal you will know your problem is on the fronts, release the clamps one at a time and see if you can pin point a side,I have had problems in the past with new/reconditioned callipers with the seals being to tight, and retracting the pistons back to far, I took out the pads one at a time and pumped the pistons out about half way, then pushed them back in , I did this a few times to each piston, and that sorted the problem, if the pedal is still not good with the fronts clamped off you will know your problem is elsewhere in the system
Andy Tilney

Follow Andy's elimination system above. I have Princess four pots on my V8 conversion. There should be no problem if they have been properly "converted," as the overall piston area is little different from standard B callipers.
Will also be worth checking if there is excessive "run out" on your Pug discs, which will cause the pistons to be retracted too far.
Allan Reeling

Thanx for input.
Andy I have silicon brake hoses (you need metric tread for Prices calipers) so am not shore if I can clamp them as rubber one? But anyway I got the picture. Pistons are otherwise flat with calipers (when no oil pressure). Also I use conversion few years just never find time to sort that out. Pads and discs have still plenty of meat. Will try to sectionalize problem but for shore it was not three before upgrade Ė but again that does not predict anything.
Aland, donít V8 use dual line brake cylinder with 7/8 inch piston?
Toni Kavcic

if you have braided hoses you cant clamp them off, which is another good reason not to have them,what you could do is push the pistons right in as far as the will go, and block the pistons so they cant move out at all and try your pedal as long as the pistons cant move, this will prove if this is the fault,
Andy Tilney

Chrome bumper (mine) V8's use the same M/C with single line system as the standard B, the only things that changed were the thickness of the disc with a slightly bigger brake pad. The later Rubber bumpers still used the standard B system.
The advantage of using Princess items is that the pad area is greater without a consequent increase in piston area which would involve a different M/C and probably rear cylinders too.
1. Have your 4 pots been converted to single line operation?
2. Have you checked the runout on the Pug discs?
Allan Reeling

Can't speak for those calipers but on my roadster and V8 I learned a long time ago that a two-stage process is required to get all the air out.

But first it is possible to determine whether the problem is air or not with the standard MGB master cylinder. It has a slow-return valve on the outlet, and this means that if there is air in the system the brakes can be pumped up to a high and hard pedal with two or three strokes. If you can do that, and then with the pedal released for a few seconds. the next push goes long and soft again then the problem is air. EeziBleed is OK for filling and basic bleeding, but it then needs a high-pressure bleed to get the remainder of the air out. For that I get the Navigator to press as hard on the pedal as she can, then I rapidly open and shut each caliper nipple in turn. The nose does need to be higher than the rear for bleeding.

If it's long and soft even with quick pumps then it's something else.

I've a '72 with Princess calipers, V8 discs and single line master cylinder. When I did this conversion initially (about 15/20 years ago)I was told that the travel on the brake pedal would be slightly longer, because ideally the master cylinder should be slightly larger. However, in practice the brakes work fine.

When you've tried everything else, sometimes leaving a block of wood jammed between seat and pedal overnight can force out the odd recalcitrant bubble.
Peter Allen

I am short with time but can provide some answers.

Andy: silicone hoses are not easy to work with so still didnít locate problem. But I checked pistons and they looks OK are not retracted

Allan: I donít know what do you mean with properly converted, I just followed description in Peter Burgerss book. I converted callipers to single feed line. First, I have simple connection made of copper tube. Later, assuming that this short link maybe contains trapped air (it was above bleeding nipples) I changed it adding 90 deg elbow with bleeding nipple on top but obviously that was not a solution (see picture).
I did check Peugeot discs and runouts are clear of pads area, pads equally worn and discs are as new.

paulh4: If I pump pedal is hard, also brakes do their job, the only thing I donít like is that is pedal travel so long.

I read again Peters book and notice interesting part (that I overlooked before): the conversion to Princess calipers maintains this ratio perfectly for all later dual circuit MGB cars. Single circuit MGBs use a master cylinder of slightly smaller bore which results in slightly longer pedal travelÖ

I googled a lot but couldnít find any data regard bore diameter of single and dual line master cylinders. My single line (square tin can) is 19 mm (3/4 inch) so just assume that has tandem cylinder bore of 7/8 inch. But still book mentioned slightly, I would describe my situation as much longer travel. But during front conversion I changed also rear brake cylinders with ďupratedĒ, slightly bigger from GT model (Moss no: GSW 1101). So maybe is combination of both modifications reason for long pedal travel. Will try to disconnect rear brakes to see if that brings any improvement.

Toni Kavcic

Toni - doesn't answer what I'm driving at. Is the pedal just as long after two or three quick pumps? Or is it higher? If higher then it's air. If just the same then it's either poor adjustment of the rear brakes, mechanical wear or alignment at the pedal end, or master to caliper/slave ratio. It could also be incorrect parts in the master, such that the push-rod and piston is having to travel further than it should before it closes off the bypass port. Until it does that it will not develop any pressure. However if only the calipers were changed it's unlikely to be a problem at the master end.

Paul: I want to be shore so I took off master cylinder replaced seals and put all back together. Asked brother to bleed while I was pumping pedal. After bleedings was pedal firm, all looks fine. Went for a short drive and pedal travel was ok. Next morning, long travel again?! Since I couldnít get help I decide to bleed with gravity (made few short slow pushes of pedal and then wait). Started at rear LH, then to rear RH, all good. Moved to front LH, no problem but on front RH I noticed something strange. After short push of pedal clear oil come out but after 15 second a burst of air bubbles (picture). I thought that hose on nipple is not fixed ok so I repeated but again same effect. First clean oil but after 15 or 20 second burst of air that after few seconds stop.
I searched for assistant and ask kid from street. I was at calliper but getting air out of line was not possible. Doesnít matter how long I was bleeding air was always in oil. I checked calliper, no evidence of oil leaking (also in past I never had to refill master cylinder). Tried to tighten all fittings but all was OK. I have problem to explain that to myself but it looks that always when pedal is released somehow air comes in clipper?

Toni Kavcic

Hi Tony - that's a strange one.

The only place where air might be drawn in under normal circumstances is the master, when you release the pedal. As the piston comes back after having expelled fluid from a caliper or wheel cylinder, it has to be replaced with air from the reservoir. That creates a slight negative pressure which might suck air in past the secondary seal, if fluid can't flow down from the reservoir.

If the air is only coming out of the RH front, then carefully check all the joints in the hose and pipe-work back towards where that pipe joins with something else. This would normally be the manifold on the RH inner wing, so only a very short pipe run.

Is this RHD or LHD? Not sure where that would take us, so just for interest.

When you replaced the seals were both pointing in the same direction i.e. away from the push-rod?

As Paul, Allan and Andy and most others know I am not an expert in anything so I might be wrong with following.

Any chance air could be coming in from bleed nipple screws? A bit of grease (red if you have it) grease around the top of the nipple thread should create a seal.

Or bleed nipple or its seating is at fault?

I had trouble with nipples seating correctly allowing fluid leaks, I found the nipples needed nipping up a couple of times after driving the car (a Spridget) despite feeling I had tightened them enough, not the easier place to fully use a conventional spanner.
Nigel Atkins

Paul: It is LHD but as you said 4 way manifold is on RH wing.

Nigel: I have same thoughts but if that would be a case then should oil shoot out when was oil pumped and then released. But that didnít happen. Oil comes out (under high pressure) only trough nipple and hose, and it was also full of air.
Toni Kavcic

That's a huge amount of air. I know you can get some weird effects with pressure and flow, but I'm struggling to think of where that much could suddenly come from.

Have you tried continuous bleeding i.e. from a gunson's EeziBleed? That would eliminate the pedal effect. Be prepared to run a lot of fluid through.

Also I'd be inclined to 'bleed' the hose taken off the caliper and see if you get any air then. And maybe even temporarily replace the braided hoses with rubber so you could clamp them off. 'Clutching' and 'straws'.


Strange fault. I agree with Paul, the fluid route up to the caliper connection needs to be isolated to narrow the search.
Chris Woodfield

I am not a fluid engineer, or even know the correct title for such a person(!), but I know you can have air suck in on very small leaks that are too small for fluid to escape. With mine the brake fluid leaked when the car was driven and parked but not at bleeding. I do a one-man bleed and try to press the pedal as little as possible.

Whether you can isolate it or not two systems that sometimes work, with perhaps smaller amounts of air but still worth trying as they cost nothing, are driving the car to shake the bubbles up and have the brake pedal held down at least over night.

Something that it seems only I have had in all of history was a reservoir cap without a vent hole, that caused an air lock on one of the calipers on mine, I heard the pads clonk. The vent hole wasn't just blocked it had never been punched out at manufacture, I had never previously noticed as the cap must had been on the clutch before and I had swapped them over without realising when I was topping up fluid levels after fluid changes on each.

Nigel Atkins

When bleeding is this being done with the reservoir cap on, of off?
Allan Reeling


I was demonstrating to my mate a 3-second clip fitting with my eyes closed and I stabbed my finger with the screwdriver

... I didn't really

... close my eyes. :)
Nigel Atkins

Thank you for all input, in MG world you are newer alone. Problem solved and somehow was problem air trapped in caliper. I took of caliper and dismantled it. Caliper is made of two halves and on top and bottom is hole (with seal) that allow brake fluid to migrate to pistons on outer side. Those holes were filled with sludge that mostly remains on not completely hardened color. After I cleaned all passages and reassemble caliper problem gone. I dismantled also other caliper and it has same problem but in much smaller scale. That is the strangest that I saw and have no Idea how it comes in calipers. Could be that contamination happen during assembly of calipers (professional company) or was maybe in brake lines (my amateur job) and at the end finished there.

Toni Kavcic

Looks distinctly like red rubber grease, used on assembly but it usually doesn't hang around like that!
Allan Reeling

Well done.

This thread was discussed between 03/06/2018 and 25/06/2018

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