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bn4 drum brake issues...any ideas?


Senior Member
Hi folks...I've been working on a bn4 with stock drums, and after replacing the rubber flexible lines (2 front and the 1 at rear axle), all wheel cylinders with new (4 fronts, 2 rears), newly relined shoes, turned drums, new Moss master cylinder(bench bled it before installing)....I still can't get a firm pedal. At first it felt like I had a bad master cylinder, so I tried another one, still have the same issue. I've bled the brakes multiple times. I've checked for leaks and have none.
If I push it slowly it goes all the way to the floor. It was suggested to me by some Healey guys to make sure the eccentric adjusters were correctly positioned, and they are. My next step is to replace the brake switch and see if perhaps that helps.
Any suggestions are appreciated!


Just a thought, isolate each brake section by clamping each flexible rubber brake hose in turn. There are special brake lock-off clamps that mechanics use but a carefully placed pair of locking pliers (vise grips) with a heap of electrical tape on the jaws works well too. Don't need to tighten the pliers very much, certainly nothing like gripping a broken bolt to remove it. The pedal will return to normal if a fault exists in one of the wheel cylinders.

Sinking pedal is most often the recuperating seal leaking in the master cylinder. This is a relatively common issue particularly if rubber particles from the inside of the flexible hoses has found its way back up the pipe into the m/cylinder. Was the fluid in the m/cyl black originally? I have found the rubber particles very sneaky and difficult to get out of the lines.

I played this game about six months ago with my Zephyr which has very similar braking system to the BN4 and ended up renewing every brake hydraulic component including replacing all the lines with cupronickle and all hoses with Goodridge stainless steel. Then I changed over to DOT5 silicone brake fluid as it doesn't eat paint and is more suited to casual use cars because it doesn't draw in moisture from the air. Results are excellent.



Darth Vader
Hi Austin,

John Sims at www.healey6.com experienced your condition when redoing the full brake system on his BN6 Healey. Over a very extended period, John changed every component on his car and could not get a firm brake. After using, what seemed to be, gallons of fluid over multiple bleedings with the assist of multitude mechanisms (including 2-man manual), John even re-rebuilt, and then replaced, his master. No luck. John’s pedal continued to sink, at best over days, but sink it did.

Finally, after massive levels of frustration and replacement of all, and I mean all, components, John brought his car to Mike Kusch, a British Car Race Mechanic. An analysis and initial bleed and brake adjustment seemed to release air that could not be expelled prior and set the pedal to finally stay solid.

My conclusion, John’s very careful drive to the mechanic seemed to have provided the needed vibration with brake activation to dislodge the trapped air bubble. A simple bleed and brake adjustment were the finishing acts.

Although I do not recommend a drive with questionable brakes, trapped air in an all-drum car has been a problem for many who have reconditioned their brakes and has cost frustrated owners quite a bit in unnecessary replacements.

Drop John a line at his web site. I know he will have much to say on this matter.

Good Luck and all the best,
Ray (64BJ8P1)


Jedi Trainee
I use a good old Ezi-Bleed kit to bleed the brake system. I apply only about 12PSI to the system and this is gentle enough, yet positive enough to gently expell any air bubbles from the entire system without having to pump a pedal.
This always seems to work very well on all series of the Healey. I am always using DOT 5 Silicone fluid which can be more difficult to expel all the air, and it still works great.


Jedi Knight
Country flag
If the pedal is not firm there is either an external leak that you would see, an internal master cylinder leak that you would not see, or air in the brake line (assuming the shoes have something to push against and your brake lines are not made out of sponge rubber). Don't ask me why, but in my meager experience my BN2 brakes are among the hardest to bleed completely that I have come across. Getting those last bits of air out, whether using pedal force or pressure bleeding from either end is just not easy. It isn't rocket surgery, but it might be brain science.
Country flag
Completely empty (disc brake) calipers can also be difficult to get that last bit of air out of. I agree, that it takes a couple of trips down the road, and rebleeding to get a solid pedal.

I'm currently in the middle of a total brake system overhaul on a MKIII; new ss pipes, new RWCs (wrong ones fitted), rebuild kit for the master cylinder and calipers (w/new ss pistons), plus overhauling the original servo (fingers crossed) that was taken out of service quite some years ago. Also new clutch m/c and rebuilt slave. I expect to be bleeding the hydraulic circuits for days...


Senior Member
Thanks all for the ideas, and it's somewhat comforting to know that others have experienced similar issues.
Andy- I had your same thought about isolating each brake to find the culprit. I did clean out the reservoir and must have ran a gallon of brake fluid through the system during my bleeding process (both brakes and knuckles!).
Ray- thanks for the info about John Sims- I have admired his site. It may be a trip or two down the road will help...it's worth a try.
Richch- I have an ez-bleeder but prefer the old-school method. Maybe I should think twice and try it.
Randy- sounds like you have your hands full, but from what I've seen of your projects and abilities I'm sure you'll get it quickly sorted out!

Thanks again all, I'll give these suggestions a try.

Deleted member 8987

When we found issues like this in shops, we tried a couple of things.
One, we would find the high point (often the master), and bleed there first, as you can and will get a bubble up high that will not work down to the cylinders in some instances.
Next, we would preform a two-man "turbulence" bleed.

You won't find anything like it in the books.

Have the guy at the wheels open the bleeder a couple of turns.
Then, the guy in the cab pulls his knee up to his chest (well, you get the idea), and NAIL the pedal.

ANY air in the lines trapped it shoved past the spot it has taken up residence.

Guy on the ground closes bleeder, you do a "nromal" bleed of that one, then move to the next.
Do all 4, and don't let the reservoir get empty.

You ain't lived until you've done on old Dodge with cylinders at 45 degrees, and the only way to get them fully bled is a feeler gauge blade slipped past the cup after lifting the boot.

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