General Tech Loss of brakes

Folks.

A friend has a Austin-Healey 100 that he has been driving for several years after it was restored by the previous owner.
Since the issue is general in nature, I thought I would also post here in the Triumph section to gleam the knowledge of this group.

My friend was driving home and lost all brakes. There was no loss of brake fluid. The car has synthetic Dot 4 fluid (not silicone).

I suspected that he had a brake lock up and over-heated the brake fluid. However, after the car sitting for days, there was still no brakes, still no loss of brake fluid, and the brake pedal still went to the floor.

He removed the master cylinder and we discovered a small tear in the plunger rubber seal. Rebuilding the master cylinder does not appear to have solved the problem. We cannot get the air out of the lines.

One thought is that air is entering the hydraulic system somewhere. All connections and bleed screws are tight. One would think that if air was entering the system, brake fluid would be leaking out somewhere, but we can not find any evidence of a leak.

It appears that the wheel cylinders were rebuilt (not replaced with new) during the restoration.

Today, we are going to try bleeding the system with a pressure bleeder.

Any thoughts?
Thanks.

A pressure bleeder would also be my next move, but...

When you rebuilt the master cylinder did you inspect the bore? A damaged or corroded bore could have caused the tear you found and the problem you are having now.

Problem could also be caused by incorrect assembly of the MC - I do not know Healeys and I do not know your experience but on some pistons the seal can be fitted backwards.

Did you pre-fill the MC, or try bleeding right at the MC? Sometimes if they are full of air, they won't pump at all.

Also make sure the MC piston is returning all the way out, not being held slightly depressed by pedal adjustment, binding in the pivots, etc.

I see the Moss website has a comment about some replacement MCs being supplied with the wrong pushrod; although that seems unlikely since it was working before.

PS, just because we were talking about it on another thread: Is the wavy washer present and accounted for? Apparently some rebuild kits don't have it, and the MC might not work without it.

Thanks for the replies.

The master cylinder bore looked fine. The only pitted area was outside the fluid area, near the push rod end, which is typical. I am sure the seals were installed in the correct direction.

We bleed the master cylinder on the car. We also bleed at the 5-way connector, which is a short distance from the master cylinder.

We can pump up the system enough to hear the front brakes pressing against the drums.
I bet we ran 20 ounces of fluid through the system, and still have air.

This type of master cylinder, which is totally different than later Healeys and TR 3s and TR 4s, does not have the wavy washer.

After many similar failures with auto hydraulics of all types, I no longer rebuild any cylinders which can be bought. Case in point, if you see pits in the cylinder, there are many more you cannot see readily and can cause issues.

If I must rebuild a cylinder, because it is no longer available, I send it to get sleeved with brass or stainless.

I might play around with rebuilding a clutch system...but a single stage brake system is not worth it to me.

About getting air in the system...I do not know what style brakes he was running...but the later Jaguar (and 60’s corvettes for that matter) will suck air into the disc brake calipers if the rotor has any runnout. I think the pulsing from the runnout at cruise will suck the air past the seal, without allowing a fluid leak once the system is back under pressure. I had old timers tell me about it...and I didn’t believe it until I saw it for myself!

John I agree get a new one. John can you go into more detail about the runout? Are you saying the pads have pulled away from the rotor and the vibration of the pistons is allowing air to enter into the system from the top?

RonR said:

This type of master cylinder, which is totally different than later Healeys and TR 3s and TR 4s, does not have the wavy washer.

Click to expand...

Ah, so this is a BN1 or BN2, not BN4 onwards.

What happened with the pressure bleeding?

Not really relevant, but I'm curious : How does the valve that prevents fluid flowing into the reservoir get opened and closed?

Randall,

Yes, BN1 and BN2 represent the initial designation in the serial numbers for an Austin-Healey 100. I forget if this car is a BN1 or BN2. Maybe if my friend will ever sell it to me I might remember.

Pressure bleeding is set for later today when the guy with the pressure bleeder is available. I will report the results. I am not optimistic. I still feel it is a master cylinder issue.

Regarding the master cylinder, in addition to the seal that is part of the piston used to create the internal pressure in the hydraulic system, there is an additional "chamber" that is "opened and closed" by a seal located near the opposite end of the piston. It is moved by the buildup and release of pressure within the master cylinder, thus controlling the correct flow of fluid and relieving pressure when the brake pedal is released.
I know this description stinks. You might be able to get a better idea of how it works by looking at the Moss catalog, which shows the internal parts of the master cylinder.

sp53 said:

John I agree get a new one. John can you go into more detail about the runout? Are you saying the pads have pulled away from the rotor and the vibration of the pistons is allowing air to enter into the system from the top?

Click to expand...

It is common on the cars that have pistons on both sides of the rotor. I don’t understand why it happens, but I feel it is a result when the rotor continually pushes the pistons back and forth, causing a negative pressure on alternating sides. I once felt that disc brakes were self compensating for rotor runout. After seeing 3 Jags and 4 Corvettes I owned suck air until I fixed the runout...I am now a believer.

Hmm, sounds more like the Lockheed master used on TR2 and early TR3 with front drum brakes. Appearance is totally different, but operation sounds similar.

Thanks for the description.

There are some suggestions on bleeding 100 brakes regarding the brake adjusters, my 100 had front discs from a later car, so I never had to figure this aspect out. Here is a link to Michael Salter's article on the subject. https://www.netbug.net/blogmichael/2017/08/21/bleeding-austin-healey-4-wheel-drum-brakes/