Unsticking brakes 411-S1

[Claude]

Rebuilt 2 miles and 18 years ago, the 411-S1 rear brakes have become sticky from inactivity. Now that I have sorted the overheating (rebuilt the radiator), it's time to move the car. But moving tends to be slightly dragging, and jacking up the rear wheels, it varies... sometimes easy to move, but hard pressing of the brakes tends to find they do not release.

I've been through the forum looking for hints, but appreciate if someone can guide me through the best way to get them less sticky. Do they require removal and refurbishment, or is there a less complicated way to get them working smoothly? If removal, are there tips that would be helpful?

How are the rear calipers removed? I read somewhere that it is more complicated than just unbolting. And if it is not completely locked up, but tends to stick and unstick, is this something that can be lubricated and unstuck, or is it a candidate for rebuild.

All advice welcome.

[Kevin H]

Claude,
If your brakes haven't really been used for 18 years, and you have been using mineral brake fluid, they may need rebuilding again.

Mineral brake fluid is hygroscopic - it absorbs moisture from the atmosphere, so it's possible that the skirts of the pistons have corroded, like the attached photo (this is what the pistons from the front calipers in my 411looked like).

There's really only one way to know for sure, take them apart and have a look.

This is what they look like
Resources - Bristol Cars - Owners and Enthusiasts Forum

Kevin

[Kevin H]

Claude, further to my previous post, regardless of the reason your brakes are sticking, if the fluid has been in there for 18 years, the system should be drained, flushed and refilled.

[peter dowdle]

I recently had a similar problem with my 411 mk 2. The back brakes would lock on intermittently . Believing the problem was the calipers I removed them and had them reconditioned. New pad for brakes and hand brake , all should be well. No such luck no change so I looked at the master cylinder. Great difficulty in getting a repair kit but eventually tracked one down in Scotland but it had missing seals which i sourced in Melbourne Australia. The sticking brake problem was gone but other problems lead me to discard the original cylinder and update to a mk 3 unit ( From early Range Rover). Some modifications needed but the brakes are now perfect.

[Claude]

On the Bristol Owners Club forum, Roger and Ken both correctly identified the problem, or I should say, nailed at least one problem, there may be more after I address this one. I will repeat my documentation here for the edification of future owners.

The flexible hose by the rear axle is completely blocked. Annoyingly, it looks ancient, whereas the metal lines are all new. So much for paying shops to do the job.

Curiously, the rubber hose looks almost identical to the one I removed from a 1969 Alfa Spider - same thread, same length. Tomorrow I will head to town and buy a replacement rubber hose.

After that, I will need to check calipers, servos and the master cylinder.

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DOCUMENTATION: For future 411 (or similar) owners with brake problems like mine:

Supplies:

• New hardware store oil can (metal tank with pump squirting lever and 1/4" solid tube) See https://www.amazon.co.uk/dp/B000LFTT5Q for an example
• New hardware store 6mm clear plastic hose (3 metres) and several hose clamps to fit
• New brake fluid
• Kunifer brake line (a soft alloy also known as cupro/nickel) 3/16" SAE and male & female flare tube nuts to match the Bristol thread
• Brake flaring tool that does double flare

Remove the driver's side (RHD) brake tube flare nut (male) from the T section. Temporarily you will be replacing this with a test brake tube flare nut. BTW, if you don't know how to flare Kunifer brake line, it's forgiving and easy to learn. Get the tools and watch a few YouTube videos or buy a case of beer and stop by your brake supply shop half an hour before closing time for a tutorial.

Make up the testing tool:

• Take a section of the Kunifer brake line (say 300-500mm) and double flare one end with a male tube nut that will go into the T section.
• Put a similar double flare on the other end and slide the clear plastic hose over. You will need to use the host clamp on top of the flare to keep it from leaking under pressure.
• Slide the other end of the clear plastic pipe over the oil can and hose clamp it.
• Fill the oil can with new brake fluid (this is why you buy a new oil can... no contamination)
• Attach your fluid testing pump tool to the T section. Make sure it is tight.

Begin testing

• Open the bleeder valve on the opposing side of the rear axle (left/passenger side on RHD). Attach a hose into a clear glass jar on the ground that you can see from your side.
• Pump the can. If fluid comes out on the opposing side, you know the rear axle lines are not blocked. You might as well keep pumping until the fluid is clear, you're doing the equivalent of a bench bleed.
• Close bleeder valve
• At the front end of the car in the battery compartment, disconnect the battery to avoid sparks and then disconnect the brake line leading into the servo that connects to the rear brakes and pump again. If you are unsure which servo, climb under the car and follow it from the back to the battery compartment.
• In theory this would reverse flow brake fluid forward until it dripped out (have a cup or big towel ready to catch pumping brake fluid (or make up and attach a female nut to metal line to plastic hose to glass jar)
• If it does nothing (no flow from the front brake line), remove the rubber flex hose that connects the body brake line to the axle brake line. This probably is the culprit (it was for me). Try blowing air through it. If no air, you know it is junk.
• Then hook the testing pump to the metal brake line in the battery compartment . If fluid flows out of the rear line where you removed the rubber hose, you can presume the metal line is OK.
• You can also mechanically test this if you have a new stainless steel bicycle brake cable wire that you slide inside the car's metal brake lines. It should come out the other end clean and with no resistance.

When doing this job, make sure you know where to buy replacement Kunifer brake lines and matching nuts because most cars will not come apart easily. Nuts may strip, or if frozen may twist the old metal lines, requiring replacement. A brake supply house should carry both the metal lines and the rubber hose. Surprisingly, my 1970 411-S1 hose seems almost identical to my 1969 Alfa 1750 hose. Well, maybe not so surprising, since the brake shop matched up the Alfa ones last year and found they were for a "H808 Hose Austin/Bedford/Hillman"

More reports to come, although it will be great if this and a fluid flush/bleed solves it.

[Kevin H]

My view is one should begin by bleeding the system as one would do normally, without all the surplus paraphernalia. The brake pedal along with the brake boosters produce at least 1500 psi of hydraulic pressure through the brake lines - far more than any oil can; so if this doesn't force brake fluid through the bleed screws, you know you have a blockage somewhere. If you get fluid flow through the bleed screws, then you know the problem is something else.

Also, unless you have reason to believe someone has changed the braking system, the rear master cylinder, marked 'A' operates the front brakes and is connected to the upper vacuum booster. The front master cylinder marked 'B' operates the rear brakes via the lower booster. When bleeding the system it is advisable to do it with the engine running so that the vacuum boosters are operating.

Kevin