Amazon: 1970 Amazon brakes

[packers1712]

Good afternoon all,

I hope everyone had a good Christmas, just doing a bit of brain storming, not sure I’m allowed to use that term these days I believe it offends some, my 1970 Amazon brakes have suddenly got rubbish, virtually overnight in a matter of speaking, used the car approximately a month ago with absolutely no issues, again on Christmas Eve but this time with appalling brakes more than usual travel and less than half efficiency, some fluid loss from the reservoir but no visible leaks.
My wife followed me on Christmas Eve and commented that it was white smoking from the exhaust, I’ve undone the master cylinder from the servo and it is moist in there with what I think is brake fluid, so I’m thinking the master cylinder has failed internally and fluid is being drawn into the servo and subsequently into the inlet and being burnt, causing the smoke, does this sound plausible?
If so has anyone successfully re-sealed their master cylinder or am I better off just replacing it?

Thanks in advance, Doug.

[Ron Kwas]

Doug;

Symptoms of sudden decrease in brake effectiveness, accompanied by loss of fluid with no apparent leaks, and particularly white smoke, strongly suggest failure of Brake Assist Unit (which you didn't specify, but I must presume the presence of for a '70!)...you will likely find where all your lost brake fluid went, when you remove it for service...I believe rebuild kits are available, but care and special techniques and tools are required! The large Vacuum Seal will get chewed up quickly, and lead to early failure, if the ID of vac chamber is heavily abrasive with pitting, and this is not addressed before installing a new Vac Seal...

This gentleman from the Sunbeam Tiger world, has documented the theory of operation of the BAU, plus a rebuild very well. See: https://www.sw-em.com/brake_notes.ht...e_vacuum_servo

I have yet to try my hand at a rebuild of a BAU with such a pitted Vac Ch, but have a first hand report that (multiple) powder coatings built up the pitted surface enough that it was smooth and quite gentle on the Seal when it was subsequently (and functionally successfully!) rebuilt...before I heard about the powder coating being effective, I was thinking of solder-filling (but discounted it as too soft!), or braze-filling (hard-soldering, a possible answer). The powder coating technique seems a lot simpler so I will try it, probably after filling the pitting with braze...

Good Hunting!

[packers1712]

Hi Ron,

When I use the term “servo” I’m referring to the brake assist unit, that’s what we tend to call it over on this side of the pond, how would it’s failure lead to consumption of brake fluid surely brake fluid shouldn’t be present in the servo if the master cylinder is in good condition?

Just as an additional note my car has a direct acting servo with the master cylinder bolted directly to it and the rather clunky rod assembly above the rocker cover at the back of the engine, it isn’t the remote item fixed to the inner wing/wheel arch.

Doug.

[packers1712]

Photo attached hopefully!

[Ron Kwas]

Doug;

While he describes the separate, stand alone BAU, I expect the function and rebuild info he presents would be adaptable with a little imagination (and the x-ray drawing in the service manual) to your tandem BAU.

Brake fluid leaking into the vac chamber is not a normal condition, but occurs when the Low Pressure Seal fails. See vid timestamp: 23:13

Yes, your description of how and why the brake fluid is being consumed is absolutely correct!

Cheers

[142 Guy]

It does seem like your master cylinder has 'purchased the farm'. However, I expect that you may also have an issue with your brake servo.

MC seal failures can occur in two ways. The primary or secondary piston seal fails which causes loss of pressure in one circuit; however, the brake fluid leaks past the piston and goes back up into the reservoir with no external loss of fluid. The second failure is failure of the seal at the back of the MC where the servo thrust rod contacts the primary piston in the MC. This allows leakage out of the primary circuit and in to the area where the thrust rod from the servo enters the back of the MC resulting in external loss of fluid.

I have attached the cross section for the servo on the late 120 / 1800. Having a 140, I am not directly familiar with this servo. In the diagram, the front chamber area marked 3 resides at engine vacuum. You can see the port (20) that connects to the intake manifold. My recollection of my 140 is that where the thrust rod (4) exits the front of the servo there was a seal / bellows to block air flow into chamber 3. The diagram shows no explicit seal around the thrust rod so I am not sure what is going on there. Perhaps they rely on MC flange where it attaches to the servo housing to provide an air seal?

If there is no explicit seal where the thrust rod passes through the front of the servo housing then a leak from the back of the MC primary piston could allow brake fluid to travel along the thrust rod and into the front chamber and then possibly get sucked into the engine. The thing is that normally there is no air flow through the brake servo so I am unclear as to how the brake fluid would carry from the servo into the intake manifold to be burnt. If you do have an air leak in the servo then this may allow for fluid transport into the intake manifold. I would also expect that you might be suffering from a high idle if you have servo air leak.

Short answer. I expect you have a failure of the rear seal on the master cylinder. If you had a piston seal failure you would have to hone the MC bore out when you install a replacement piston. However, you don't have a piston seal failure and it might be possible to replace the rear seal without touching the piston. If you have to remove the primary piston to replace the rear seal chances are that the old piston will not reseal when you re insert it.
If you have to replace the primary piston A to replace the rear seal the MC cylinder requires honing. Personally, if I have go through the work of removing the MC and bleeding the brake system I would just install a new MC (if they are available) rather than attempt an MC rebuild.

The little longer answer is that you may also have a problem with the servo. If you have something like a Mityvac you can connect it up to the vacuum port on the brake servo to see if it holds a vacuum. If it does hold a vacuum then the servo is probably OK. If it doesn't then I think you need to investigate that further.

[packers1712]

Thanks gents,

142Guy, I’m struggling to fully understand the concept of, if like you say and I suspect, the rear master cylinder seal has failed and fluid is able to enter the front chamber why wouldn’t it be drawn under vacuum into the inlet, what would stop it?

I’m not saying I haven’t got a servo fault too as I have had a symptom in the past where when I’m pressing the brake pedal for a long time, motorway traffic slowing from high speed to a gradual stop, I find I need to press more towards the end of the stop, which make me thinks when the engine is off throttle it can’t keep up with the vacuum required, perhaps due to a leak?

Doug.