Binding brake

[Philip]

Hello,
My front left caliper is binding on.
The caliper itself was a new old stock item fitted a couple of years ago.
When the car is driven only a short distance the front brake binds.
I jacked it up and took the pads out last week and both pistons are free but can't be pushed back at the same time.
However, if I undo the bleed nipple, both pistons can be pushed back.

It has single circuit brakes, the lines seem to be clear and I don't think the hose has collapsed.
If I pinch the hose hard I can feel it pulse as the brake pedal is applied and released...

Is there any kind of valve in the caliper that could fail and account for this behaviour?

Cheers
Phil.

[MarinaCoupe]

There's no valve in the caliper, pistons can stick, look for marks on the side of the piston. My other thought is that the flexi-hose to the caliper may have collapsed if they are any age. The caliper sticks because the rubber hose won't allow the brake fluid to return up the hose. How old are the hoses and are they trad rubber or are they modern nylon with stainless braiding?

[Philip]

Hi Chris,
The pistons are both free, you can move them with your fingers. One at a time if the bleed nipple is closed, or both at the same time with it open.
The hoses are rubber. I have no idea how old they are but they appear to be in good condition.

Are brake hoses still readily available?

I will have another look this weekend if I can.

[mickthefitter]

I haven't yet had the front wheels off and looked at the front brakes of my own current Morris Marina (the last one was in 1985) but am I right in thinking these are the commonly found single piston floating calipers? In the past I've found dust and corrosion can cause the pins that the caliper floats on to stick, causing the 'fixed' pad to remain in contact with the disc. Alternatively I had a BMW in the late 80s where the brake master cylinder developed a step in the bore, holding the brakes on. Where the caliper pins have stuck, I've sometimes had some success (where the metal pin can be seen if there's a dust cover to peel back) by taking the pads out and using WD40 and work the caliper in and out carefully with a soft faced mallet. Ideally then use brake grease, high melting point grease or copper grease on the pins when everything moves.

[MarinaCoupe]

The brakes are old school cast iron opposed piston Girling Type 14LF, common with Triumph Dolomites and Spitfire
1500.

Here's a pair on eBay.

http://www.ebay.co.uk/itm/Morris-Ital-M ... 2866409415

IMG_0109.JPG (27.68 KiB) Viewed 8066 times

IMG_0108.JPG (28.15 KiB) Viewed 8066 times

[mickthefitter]

Twin pistons. I never knew! In which case, discount my theory!
Most of the brakes I've worked on have had floating calipers, only a Merc I owned that I worked on myself had twin pots. Back to the collapsed hose theory then

[MarinaCoupe]

Mick does that mean that you've never had the front wheels off yet?

[mickthefitter]

No not on mine! Now when I first got it and began to drive it, there was a bit of moaning from the brakes odd times on corners, and I suspected sticking calipers from lack of use, but it went away and the wheels never felt hot. Then during the winter lay-up, I was worried about the calipers but I've had the front in the air for other work, pumped the brakes, and the wheels still spin freely. But no, I've not taken any wheels off. Its come back to me that in the 80s I did change the pads on my Mk1, and had no real trouble (I struggled to get the pistons to push back on my Merc) but the pads did rub afterwards on full lock. My foreman and mentor back in the 80s where I worked, reckoned the flexi-hoses were just putting pressure on the fluid when stretched, and it was nothing to worry about, so I left it. Mind you Ken did drive throughout winter with 5 litres of anti freeze in his Viva boot rather than in the radiator, and started using oil from a drum in the factory in his engine, instead of motor oil!

[Philip]

I had another look today.
Bled the brakes.
There was an improvement but it's still grabbing...
Caliper, I am 99% sure is ok.
I'm fairly sure the hose is ok too...

But, it can't possibly be the master cylinder as none of the other brakes are affected. I don't see how it can be a metal brake pipe, so really, the weakest link by default is the brake hose...

Anyone have any other ideas??

[balmy]

Have you checked the pads move freely? Sometimes they stick.
You still is Swindon Philip?

[mickthefitter]

Not really. That Beemer I had with the faulty master cylinder was inclined to put one front brake on more than the other, but I wasn't aware at the time of any issues with the rear drums. I'd had someone (a BMW independent) fit new discs and calipers, as that was his suggestion, and the car was just as bad afterwards. So it went in again before the new discs got wrecked and the master cylinder was the fault. Expensive car. I'm not sure if a faulty hose will behave the same as the Beemer did with the faulty master cylinder, but what I did was jack the front of the car up, put it on axle stands and left it overnight with the front wheels hanging. In the morning the pressure in the system had dropped and the wheels spun well. One pump of the brake pedal and the wheels were tight. I guess if you did this and one front wheel was as free as a bird and the other tight after pumping, you've got one problem on one corner, caliper or hose. If BOTH wheels are tighter, even if one is more so than the other, I'd think it points to something else....

[Philip]

Hi David,
Yes, I am still in Swindon.
The pads are as free as you like.
I couldn't push the pistons back at the same time unless the bleed nipple was undone. With the nipple undone I could push the pistons back by hand.

Hi Mick,
I don't think master cylinder can be the issue because my car does not have split circuit brakes. The master cylinder only has one outlet.
I think my next step is to replace the hose, but I struggle to see how it can be the cause.

[mickthefitter]

I think they can swell internally. Foot pressure overcomes it, but the narrowed internal bore can prevent the small amount of piston retraction from the rubbers from taking effect.

[david painter]

The Most likely Culprit is going to be the brake hose. Providing pads are free and you can push pistons back with bleed screw undone shows caliper and pistons are free. Flexi Brake hoses are available and reasonably cheap. If it was my car I would change it before going any further.
Dave.

[MarinaCoupe]

Ditto David,

What happens with a collapsed hose is that the master cylinder (m/c) pushes fluid down to the caliper piston and then as the spring in the m/c pushes the m/c piston back, it creates a vacuum which sucks on the rubber hose which collapses and closes. As the m/c reservoir refills the m/c, the caliper now has nowhere to displace the fluid so it can't release the pressure so the brake pad(s) bind on the disk brake. It works exactly the same on drum brake flexi hoses.

Brake flexi hoses are some of the hardest working pieces of rubber in a car and unless you know when they were last replaced they could conceivably be the original ones, even if they look OK on the outside they will deteriorate on the inside.

There is separate debate on replacement rubber or stainless steel braided nylon hoses, rubber is fine, but if you are buying off eBay the hoses could be 10+ years old already, whereas braided nylon should be relatively new and will last longer in any case.

Also as David implies a set (or single) hoses will be a lot cheaper than either a master cylinder or caliper, so change them first before changing anything else.