brake problem: right diagnose?

Certainly a possibility, if the rubber hose is disintegrating inside, it could have bits acting like a one way valve. Worth checking too that the pistons haven't started to jam up that side, and that the calliper isn't getting a build up of rust and crud that might make it harder for the pads to slide off the disc.

Could be worse... Giving a g/f a driving lesson in a large car park one evening, a police car turned up. "Driving lesson?" he said... "How could you tell?" I replied.. "Get out the car and take a look" he said, with a grin on his face.

She'd been dragging the brakes all the time, to the point they were glowing orange yellow, and thick columns of smoke were rising from both front wheels...

Certainly. I've experienced this problem. New hose cured it.

Allan Bennett

To check whether the hose is ok:

When the brake is holding on, slacken the bleed nipple, if the wheel is immediately free then the fluid cannot get back up the brake hose. I have only come across this once in twenty five plus years of repairing.

MrCheerful

IMHO, it's an urban myth. Neither my brother or myself have ever come across it in countless years of running bangers. Hoses tend to deteriorate on the outside first - they crack and fail an MOT. So the chances of a flexible flap developing inside are remote - unless it was like that from new, a manufacturing defect.

the

applying

stopping

arch, and

wheel: there

caliper.

new

too).

(swelling of the

releasing

the brake

yet..

I very much doubt that the problem is caused by the brake hoses.

I assume there is clearance between the operating rod and piston.

Did your service include replacing all the seals in the master cylinder? If not, I would advise you check the master cylinder seals, especially the one at the rear, nearest the operating rod. and on the cylinder itself, that the small bleed hole from the cylinder to the reservoir is clear. If this gets blocked, you can get the symptons you describe. The main seal only just uncovers this hole with the piston in it's return position. An old swollen seal can cover the hole, slowing the return of fluid to the reservoir, or even stop it altogether. Mike.

Like others have already told you the odds of it being the the hose are about 50 million to 1. As you have already changed the seals the most likely causes

(1) Pad sticking in caliper (2) Hydraulic lock caused by piston in mastercylinder or servo not returning fully --- check the pedal & its linkage are returing fully. (3) Vent hole in mastercylinder resevoir blocked (4) bent caliper bracket

A scientifically ascertained and verifiable ratio?

Or are we to accept that so-called experts (especially self-appointed ones) can get things wildly wrong? (eg the 'expert witness' who made similar claims about cot-deaths. And as for Hutton...)

Having been through the above, and more, and consulted with experts in the field - I eventually found the culprit to be a blocked flexible hose (the rear one) on my Dolomite Sprint.

On subsequent discussions with a trader, he recounted a similar event when a customer returned a Spitfire caliper which he claimed must be faulty. The culprit? A blocked flexi-hose.

We've had the discussion on this group before with a number of similar examples...

Allan Bennett

In my /experience/ it is NOT a myth.

Rather than that, it would seem that the rubber might degenerate where it is in contact with stagnant fluid in a disused vehicle. The resulting goo can completely obstruct the aperture.

Astonishing but true.

Allan Bennett

I still say it acting like a one way valve is.

If it's full of 'goo', why would it act like a one way valve? And why only one hose out of several?

Gee that would take a lot to bend - most calipers are attached to cast brackets. Or is the caliper loose (along the same lines of thought)

Does the Rover P4 have fixed calipers or a floating caliper? (not familliar with these)

Pressure I would've thought. Think of how much pressure can be exerted on the system when you press the pedal, then how much is going the other way when you let off. Only needs a slight excess in one line to cause binding on that disc, and have a cooked corner after a few miles. And one line because the relative travel of fluid isn't going to be that high, so unlikely any crap will find it's way back to the m/c and into the other lines, especially as said crap is already preventing brake fluid moving as far as it normally should!

It is possible for just one line to be faulty. And if you think about it, why would a pad stick in a calliper and bind the brake? Simply because there's alot more force used to get it onto the disc than there is to get it off, so once again the extra pressure of applying the brakes negates any problems for the pads to move into contact, but when released the pads might not have enough force exerted on them to return fully.

Still worth checking the calliper and making sure the pads aren't crudded into it on this car, but it could be the hose. Even a 100 million to 1 chance is still a chance..

It's normally the piston(s) that stick on a design like this.

Well, whatever your scepticism, it happened to my vehicle. The flexi-pipe over the rear axle, which caused just the nearside rear wheel cylinder to lock up - released by opening the bleed nipple).

I think the answer to your Q is obvious, though: when applying the brakes there is positive pressure forcing fluid along the system (especially when this is servo-assisted); on cessation of braking there is no pressure forcing the return. But you know that...

Allan Bennett

All it needs is a hole the size of a pin hole to vent the pressue back to the resevoir, the pistons are pulled back a tiny ammount through a combination of the elasticity of the inner seal and knock back from the disk. Usually the seals just harden with age and can no longer pull back the piston -- but in this car that should be the case as the seals were changed. So that leaves either something mechanical (sticking pad) or a hydraulic lock at the servo (if fitted) or mastercylinder.

To be honest I can't remember ever seeing the front brakes on a P4, the rear drums on the other hand I remember well as being Girling similar in pattern to those on some Humbers, Commers and the Austin Champ, so I suspect the calipers will be conventional Girling similar to those fitted to the series Humber Hawk --- 60s Girling calipers are quite prone to sticking due to corrosion scale build up on the bore of the caliper between the inner seal and the dust cover.

A lot of cars in the late 50s and 60s such as some Triumphs and the Marina had bolt on fabricated brakets for the calipers it is quite common to find these bent (caused by a seized caliper piston on one side of the caliper) but this usually first shows up a spongey pedal and unequal pad wear.

Think it's before the days of moving calipers - IIRC, the first example of this was the swing caliper rear ones on a P6 or Ford MkIV.

Strange it should cause just the one cylinder to seize?

But why doesn't that pressure simply move the gunge? I've not seen any type of valve that acts on this principle.

Well that proves it then.

I've got one of these hoses. Came off a Mk 4 Cortina and I was so surprised by it that I kept it. Since then I've discovered it's not that unusual.

If it was a '70s European (single piston. sliding caliper) I'd look at corrosion of the caliper stopping it sliding freely. In a Brit though, with pistons either side of the disk, I'd check the hoses first. Even if one piston does stick on this type of caliper you only get enough drag to burn a pad, not to seriously deccelerate you.

And why _would_ anyone do a major brake rebuild and not replace the one component that's regularly flexed, out in the weather and made of flimsy rubber ?

-- Die Gotterspammerung - Junkmail of the Gods

What else can it do ? Pressures are high, sidewalls are reinforced. Just about the only failure mode you've got left is to have the inner liner delaminate. Application pressure is high enough to blow most obstructions out of the way, so if a one-way valve does develop, it's the only way it can face.

I've certainly seen more rubber brake flexies develop an outer wall aneurysm than I have one-way valves, and even more simply shed loose crud, but it does happen. As to blockages, I've only seen this once and it was caused by poor fitting and not holding the hose end from rotating when fitting the tube nut for the rigid pipe - easily noticed whilst first bleeding the brakes.

PTFE hoses are a real maintenance hazard, which is why they're not type approved as a retrofit on a road vehicle. They have all sorts of nasty cold flow behaviour that can cause a leak, even after fairly insignificant external damage. If you use them, don't skip their inspection (and use the Moquip hose with the PVC overjacket over the braid, not Aeroquip).

One goes first. Drive around long enough and you can probably collect the set. Off you go then.

-- Inbreeding - nature's way of always giving you enough fingers to count your cousins