Bleeders for XK 120 early type

Seems this has morphed into another glycol v silicone brake fluid conversation, so I’ll add my two cents.

After a 2-3/4 year total restoration of my E-type I went with DOT5, only because I’d ruined the paint on a door years earlier with DOT3/4. I’m still using DOT5, but only because I don’t want to do a complete brake system rebuild.

It was difficult bleeding the system because DOT5 has a propensity to entrain microbubbles. It’s advised not to use the stuff right after purchase but to let it sit on the shelf for several days to allow the air to come out, but in my experience that isn’t long enough. Weeks, maybe. Instead I extracted the air in a vacuum chambre, which seemed to work, but I was never able to achieve as firm a peddle as I had with DOT3/4, and that’s the way it remains after 8 years. The brakes otherwise work fine. I’ve had to lock up the wheels a couple of times during idiot encounters. No problem. But I still don’t like the softer pedal feel.

I filled and bled my XK120 brakes just last week using DOT3/4. I never even considered DOT5 for it. Its brake system is more primitive and without a booster already requires considerably more effort to slow and stop the car - I definitely did not want to add a softer pedal to the mix.

And, contrary to conventional thinking, you still need to replace DOT5 every few years because it still absorbs accumulates moisture, albeit much more slowly. And, have you priced the stuff?

I’d advise against DOT5.

Hi John,
Nothing seems to stir the emotions more than the discussion of silicone brake fluid. Yes my brother and I successfully run it in our tandem master cylinder 120’s, my brother now approaching 20 years (over 30K miles) with no seal replacement or rebuilds. I have had it in my car for 4 seasons (4600 miles so far) now with no issues. I have heard all the stories of spongy brakes and air entrapment, and locked up brakes, but have not experienced it. After using the standard bleeding procedure of one person pumping the brakes, with the other bleeding the wheels from the furthest away to the closest, we have had no problems.

My brother discovered the key to success is to use genuine Lockheed brake seals and original tilt valves, not always easy to find, but still out there if you care to search for them. I used them in my car and all is good. No concerns about paint stripping or biennial brake flushing with fresh fluid. Modern seals and tilt valves supplied by the usuals are incompatible and swell up. At minimum, a test should be performed on new seals to see if they swell in a jar of fluid. The advantages far outweigh the perceived disadvantages in my book, but to each their own, of course. And we drive our cars regularly at highway speeds, often for hours, heating up the brake systems, which is good for them.

Tom Brady

Which is exactly why I paid him dearly for precisely these parts.

Why mess with success.


I should clarify my last statement. DOT5 doesn’t absorb water. But moisture will still find its way into the system and concentrate in localised spots. You can definitely extend the period for replacing it, but you still need to replace it after a few years. YMMV.

Sure, so how has John Brady made it to 15 years w water in his system?

It’s no hardship to let a container of DOT5 rest on a shelf for months before using.

We have a vacuum chamber at work which we use to help remove micro bubbles from adhesives and other low volatility liquids. It’s almost mandatory to use on liquids that require mixing (ex urethane molding mix).

Might be a safe thing to do to containers of DOT5 before using it.

Dry climate maybe? Here in Niagara it gets very humid, and since the system is vented at the reservoirs you can’t keep the moist air out. I was advised by a local Jaguar mechanic that doing a fluid replacement every few years is prudent, so that’s what I do. The purple colour of the stuff seems to fade with time, but I don’t think that affects its performance. Again, it works fine in my E-type. I just prefer a firmer pedal.

Why can’t we put a flexible (bellows type) sealing diaphragm on our fluid reservoir to block that moisture path? Might require replacing the reservoir

Here is one for motorcycles:

Might help. This is what they say about brake fluid in Harleys:

How Often Should You Change the Brake Fluid for Harley®?

Glycol-based brake fluid needs changing at regular intervals as recommended by Harley® for your bike. For most riders, that’s every 1-2 years. If you use DOT 5 you can go longer between changes.

The key point wrt this segway is the last sentence. I’ve changed the DOT5 in my E twice in the past eight years. Just about due again.

I considered starting fresh with DOT5, so I went through the archives to see what the consensus was. It turns out that the pans of the balance are each very heavily weighed. There are many people who think it’s the best thing since sliced bread and just as many who say to avoid it like the plague. (My apologies to the idiom police.)

Hi John,
Interesting question. I have owned and driven a 1984 Yamaha FJ1100 since 1987, I put on about 1000-1500 miles per year. It has hydraulic front and rear brakes and a hydraulic clutch. The front brake and clutch master cylinders are screwed shut and have a diaphragm that is allowed to contact the fluid and rise and fall with each operation of the hand lever. I picked this off the internet to show what the clutch master looks like. The clutch and brake master(shown) are very similar.

In1988, 34 years ago, the clutch master failed so I flushed it out, rebuilt it with new seals and used silicone fluid. That was the last time I touched it. The front brake master is also sealed in the same manner, and still has glycol based fluid in it. I have sucked the fluid out with a Mityvac once or twice over the years, but have never replaced any of the seals. There are clear windows in the sides of the reservoirs so you can see the fluid, which is still clear. I have taken the covers off over the years to check the levels and have added tiny amounts. I think the idea of sealing the master cylinder on 120’s is a good one, as per my experience.

Tom Brady

Some race cars also use bellows so seal the system but allow expansion/contraction.

Race car maintenance measures brake fluid life in terms of days and weeks.

correct…and same for ALL the fluids…sometimes hours in the same day. sometimes light oil for qualifying. Certainly, changes of engine, trans, diff after each event. Nick

Race car : Race is really hard on brakes. I flush it after every event (3 -4 hours of track time).

Sorry to hijack the thread back to its original subject, but I have run into a puzzling problem. I have replaced the master cylinder and all piping on my XK120, using double-flared cunifer pipes, and tried to bleed it with vacuum. I connected the vacuum to the left-front bleeder, where there is perhaps 2 feet of tubing between the MC and the bleeder. After passing a half-gallon of fluid through the system I was still getting huge amounts of air, more air than fluid. I connected the vacuum to the other 3 bleeders and had the same result, but at least I knew that there was some fluid throughout the system now.

I then tried the pedal pumping method. Since I have the car on a lift, I reached up and pulled the pedal down, then released it slowly, and repeated the process. I got more fluid through, but there was still a lot of air. I don’t mean bubbles; the clear tubing that I was bleeding it through would have 6 inches of air and then a few inches of fluid, and then a few more inches of air. This went on while I put another half-gallon of fluid through the 2 feet of pipe.

This futile exercise finally brought me to the conclusion that somehow air was getting into the system, because the pipes themselves can’t hold that quantity of air. I figured that I could perhaps solve the problem if I back-fed the system. That is, provide pressurized brake fluid into the bleeder and force it backwards through the MC and into the reservoir. I figured that would work since when the MC is in its released position, which it is, with 1/16" of free play between the push rod and the piston, the system is open from the brakes to the reservoir. I drained the reservoir and connected the pressure bleeder to the bleeder nipple, and slowly increased the pressure to 15 PSI while watching the reservoir. Nothing happened! No fluid went into the reservoir! I walked around the car and cracked the other nipples and got fluid there, so the pressure bleeder was working.

I know that the MC is working somewhat, since I can pump fluid with it, but a vast quantity of air is entering the system somewhere, and I can’t push fluid backwards through the MC when the piston is at full release. I don’t really want to pull and disassemble the new MC, but that’s my next step unless someone can provide some insight into what my problem can be.

The pedal pumping method on the lift should have been very easy. No need for vacuum or back pressure.
I suspect your new master cylinder. As I recall you have an early car, so probably the single system. Does it look like this?

Bubbles in the clear bleed tube while vacuum bleeding is caused by air passing by the bleed nipple screw threads. It’s why I abandoned vacuum bleeding. The bubbles never stop. I suspect that reverse pressure bleeding (new to me, but intriguing nevertheless) from the nipples is doing the same thing. There’s no other explanation I can imagine. If air was being introduced elsewhere you would reasonably expect there to be fluid leakage showing up on the floor, but that’s not the case as reported.

See above. The only way I could bleed my system - tandem m/c in my case - was to pump the peddle aggressively.

Nick, I took one of the old bleeder screws and plugged the two holes at the bottom with JB Weld. Then I drilled a 1/8" hole through the original opening at the top through the center of the tapered bottom. Now I can tightly screw this bleeder back into the brake line where the taper will seal out the outside air and allow access directly to the brake system through the new hole. No leakage past the threads, whether with vacuum or pressure.

The idea of back-flushing the system is because when bleeding is done any of the normal ways you are trying to push air downward against its normal propensity to float upward. Also, I know there is no air introduced into the system that way.

Rob, yes, that is what my MC would look like if I sawed it in half. I still don’t see how it would make an effective pump. The opening X is only open when the piston is fully retracted, so that would not allow fresh fluid to . As I see it, the only input “check valve” would be with fluid passing through the larger opening to the right of X and into the chamber around the piston. From there it would pass through the little holes around the periphery of the front of piston H, past the little flat piece of sheet steel between that and the cup , and then backwards between the main rubber cup and the cylinder, and into the chamber where the spring is. That sounds like a very convoluted and inefficient pathway, which would either take a very long time to replenish the spent fluid or would generate a lot of vacuum in the brake system when the piston returned. Another odd thing I observed with my system is that when the pedal was released there was not any tendency for the fluid to be drawn back up the plastic bleeder tube.