[Claude]

Originally Posted by 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

Yes, I agree, and did that first. When it did not produce the desired outcome (nothing came out of the bleeder screw), it suggested blockage somewhere. Of course, the fear is that the blockage is in the calipers - hard to remove and expensive to rebuild or the MC was on the way out.

So, in the absence of knowledge, I had to look at the brake system as a series of parts and work out which were the most likely to have failed. At the same time, some of those parts are hard to access and difficult to disassemble, with the risk that if, for example, a nut were to strip that could mean needing to buy and flare a new brake line with a new nut. For me each unexpected, sequential failure means cycling to the ferry, 45 minute ferry ride, walk to the car park, half an hour drive (or double if the motorway is jammed) to the brake store that permits me match up parts in the absence of a book listing parts for a 1970 Bristol 411, and then reverse this travel to get home... yesterday that meant leaving the house at 8:45 a.m. and arriving back home at 2 p.m. (I took advantage of the trip to town to do other shopping errands as well). I recite this because many owners will have similar challenges if they don't live near a qualified specialist but they don't want to wait months to sort a 15-minute repair.

If I use the brake MC as the pressure device, I (a) have no way of determining if it has failed or something else is bad and (b) there are some suggestions that I can damage it, pressing it to the floor as the piston goes into an area that could have decades of gunk that it never hits during normal use.

For what Americans call "shade-tree mechanics", the procedures are very different than repairs done by qualified mechanics in proper shops with a full inventory of tools and parts on hand. For a start, the car is not on a hoist but on stands where one must lie on one's back and slide in and out with a creeper. This means avoiding the areas where dirt falls in one's eyes (even wearing glasses that steam up) or where hands get cut when a frozen nut releases fast. Instead one selects access areas that are more accessible. It means looking for the parts that are easiest to remove without damage, or if damaged, easiest and least expensive to repair.

The problem facing shade-tree mechanics is that no one writes manuals for them. When the part is diagnosed, they can't ring up the dealer and have the correct part sent over because there is no dealer and there is no part number.

So, as I looked at the challenge facing me I saw that I wanted to avoid the delicate bits, like pistons or servos, and start with dumb bits like lines, hoses pipes and junctions. The tools one can buy to test these bits are expensive and made for shops that use them hundreds of times (and in my case, must come from overseas). However, the local hardware store does sell oil cans - the old fashioned devices that they use as the icon on the oil pressure gauge - for about NZ$12. They sell clear hose for $0.77 per metre. And I already had a brake line flare kit and some left over metal pipe and correct size nuts. The previous day I had purchased 5 litres of brake fluid for NZ$69, so I had a surplus of disposable fluid to test.

It took more time to set up the test than it did to run it, but it confirmed that the rubber hose was blocked. Next day, the brake repair shop kindly let me in the back where I went through racks of hoses and found the exact match for all of NZ$28 (to determine £ equivalent divide by 2 [£14]). It was marked H892 and the book said it was for a Hillman and a British Ford. Online they say it is for lots of British cars of the era, but no mention of Bristol.

The purpose of the subsequent documentation is to assist the next owner facing a similar problem. The task is to find easy ways to diagnose potential problems using common resources. While, for example, your suggestion of using the MC to test with the engine running does have the benefit of being a built-in tool with 1500 psi, for a shade tree mechanic it means doing the work outside, since running an engine in a tight garage is ill advised for health and safety reasons. That means potentially leaving the car outside on stands for days while one tries to isolate and repair the problem, or putting everything back together again to drive the car back into the garage and hope one has enough braking power to stop the car.

Forgive the length of this reply (editing takes more time and I do need to get back to work), but the point is to suggest that I am not alone in this challenge, and one of the great benefits of forums is that they permanently record shade-tree challenges and solutions. If each of us so documents, gradually a wealth of knowledge develops... knowledge such as knowing that a hose Bristol used was made by Girling for a Hillman, or that hoses commonly block if the car has sat for years and should be tested using a simple shade-tree-made tool.