Have spent over an hour studying previous threads about bleeding XK 120 brakes, a few questions remain because of the age of the posts. Our previous method was an able bodied assistant of 56 years (as of today) pumping the brakes while I operated the hose, jar, and wrench on the bleeder valve. There are two basic bleeder methods available today–pressurized and vacuum. (My assistant is a little less able bodied.) With which of the two methods do members have experience? What is your favorite product manufacturer? If memory is accurate, the deed can be done without removing the wheels. Correct?
Yes, it can be done without removing the wheels. The rear access is a bit tight but it is doable. Luckily I still have an able bodied assistant, thus use that method. I have no experience with the other methods I’m afraid.
Mine is the single system.
I have no assistant, so I remove the wheels and use the clear hose, clear jar, wrench, just barely crack a bleeder, pump a few times, close bleeder, pump a few times, check the reservoir, repeat. There is a differential pressure valve ABC in the master that is supposed to make it easy for fluid to flow out and slower to return. Seems to work for me.
Years ago I had a problem with seal F blocking pressure release port X, the seal was swelled up with silicone fluid. Never again.
Is the slow fluid return that which enables this to be a one person job?
Yes I use that method on any car, the suck back is always slower than the pump out, just release the last stroke slowly.
Just did this job yesterday - more precisely completed it yesterday after giving up the day before. Tandem m/c, but the challenges would be the same. I started out by pressurising the system at 15 psi and got good flows to all 4 bleed nipples but no peddle at all after several attempts and reservoir refills. I was beginning to think the seals in the new m/c might have been damaged. It took the missus getting behind the wheel and pumping the peddle vigorously while I made the rounds a half dozen times to firm things up.
Compared to an E-type, for example, it’s not the easiest hydraulic system to bleed. For one, the brake lines are 1/4” in diameter rather than 3/16”, so you’re pushing a greater volume of fluid. But the biggest difference, and why it’s so challenging to bleed the air out, is all four bleed nipples are situated at the lowest points of the system. You really need to get the fluid flowing fast to push those bubbles out of the high spots.
I filled the system with DOT4. Toward the end of the process most of the air coming out was broken up into microbubbles from the vigorous pumping. I think bleeding would be an even more difficult proposition with DOT5 and its propensity to entrain air.
Thanks, Nick. You also answered the question about choosing DOT 4 vs anything else. Do you think a vacuum system would have been better than a pressure system?
I’ve used both pressure and vacuum bleeding on my E. Pressure bleeding works well but I’ve found that once the bleed nipples are cracked open when vacuum bleeding air is introduced into the fluid stream and there are constant bubbles showing in the bleed tube. So, no, not better. However, when I installed remote rear bleeders in the E, which moved the bleed nipples from the top of the pots to below the callipers, I had a hard time getting the air out, so pressurised the system per usual, attached the bleed hose to the vacuum bleeder, put the jar under vacuum and then cracked the nipple open. The combined pressure and vacuum causes a rapid rush of fluid downward, taking any stubborn bubbles with it. May work on the 120 as well.
A diplomatic solution…use both!
I have tried all methods on my 54 xk120 but the one I like the best is to put a check valve right after the bleeder hose attachment and then a long (10 ft) clear line back to the reservoir. Once you fill the line and refill the reservoir you have a closed system and you can then pump the hell out of it and witness the flow because the clear line can be positioned in front of you.
Intriguing. Where did you get the check valve?
just google brake bleed check valve. Used mostly on motorcycles.
Intriguing indeed. You’d want to make sure the tube is well secured at the reservoir end, errant DOT 3 or 4 doing what it does to paint. But no air entrainment issues? The ejected brake fluid from aggressive pumping becomes slightly cloudy from microbubbles and needs to sit for a bit to clear. DOT5 much more of an issue than 3/4. I don’t reuse the ejected fluid when replenishing the reservoir, taking fresh from the jug. With less aggressive pumping it’s likely not an issue, though I found it was the only way I could clear the system of air.
Yes I do not pump aggressively and use only Dot4. In fact i pump using my hand by kneeling on the flor and watching for the bubbles. I also attach the hose with twist ties to the reservoir and the hood support. If you are flushing then you need to do that before you use this method to make sure you have fresh fluid in the system. This one man method is fool proof and worked well for me. Don’t forget the check valve or you will loose the fluid in the line when you change wheels.
pressurizing the XK can be tricky due to the brake fluid reservoir cap. Some have used an extra cap and made a fitting. Use only very low pressure. Some cars, (older Corvette) have just used a slight pressure and open bleeders and let it just drip…no brake pumping, a pressure/gravity flow. Time consuming and uncertain. As others said take care to have no brake fluid contact painted body. That said the ol assistant to push the brake pedal is the way to go…it does not take much pressure or much pumping. Just some time, to get 4 locations done. The fitted tube into a jar of brake fluid, with a proper wrench for the bleeder and it goes quite well. OK…an issue with pushing the brake pedal to bleed…if done yearly or so it is actually good to have the master cylinder piston move beyond its usual travel, keeps things clean and moving, but if left for a long time, and if any corrosion at all in the area of the sleeve beyond normal travel the seals can be damaged…and thus now a leaky master cylinder, allowing fluid out or air in, and a rebuild necessary. When done…be sure the brake pedal has the specified free play. I suggest Dot 4, or a fluid specified as 3/4. Good idea to flush thru a lot as brake fluid attracts moisture and over time will for certain have water in it, result is poor brake feel, but worse corrosion in the system. I am not a fan of the quick bleeders or check valve bleeders as any malfunction and braking can become disastrously poor without notice. I want my wrench hand to feel the bleeder close. At F1 Austria in practice, I think it was Hamilton, did not like his brake feel…they came in, bled the brakes…and back out. They have to be spot on to brake at 5 Gs…and not lock up. (I had not known…that the accelerator was not mechanically linked to the engine…it is electronic…with a spring to give it the feel of mechanical to the driver.
I’ve used the vacuum and pressure assist devices and the pump and bleed with an assistant. I like the results of the pumping fluid from the wheel cylinder back through the MC up into the reservoir best, as it forces the air to it’s highest point, but it is the messiest, as pressurizing the cracked open bleeder causes a leak. It’s difficult to get a strong flow using the vacuum system without air leaking in around the open bleeder. And those air bubbles contribute to confusion as to whether the air is coming from inside the wheel cylinder or around the bleeder.
I use the vacuum bleeder method because I have never been fully successful in getting a hard pedal by pumping the brakes. However, I always get bubbles of air that get sucked past the unscrewed bleeders, so it’s hard to know when to quit. I have heard several suggestions, including Teflon tape or grease, but none have been successful for me.
I just replaced the entire brake system on my XK120, from the master cylinder through all the pipes, hoses, cylinders and bleeders. Since this will be a major bleeding job I am going to try a new idea for improving the vacuum bleeding technique.
To that end, 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. When the bubbles stop I will replace the modified bleeder with the proper one.
Anyway, that’s the plan.
Consider Silicon fluid. Brady’ brothers swear by it and you know they are picky bastards.
If I ever have to rebuild my MC, I’m switching over to that stuff too.
It is not quite that simple John…to just switch to silicon: The entire brake system needs to be rebuilt, clean, flushed, and being sure the seals and cups and rubber parts including hoses are all newer, any old rubber parts will turn to goo. And also this: "There is little advantage in adding silicone fluid to a system which contains even small amounts of contaminants. Merely bleeding the system is not enough, as there will be pockets of old fluid and sludge which will not bleed out. Silicone fluid tends to concentrate any residual glycol fluid, moisture and sludge, into slugs, instead of allowing their dispersal throughout the fluid, as does glycol fluid. This can lead to relatively severe but localized problems, rather than the more general system deterioration experienced with old moisture-laden glycol fluids. This may be a factor in reports of leakage when silicone fluid is used in non-rebuilt systems which had been used with glycol fluid. "
Source and complete article by Moss: Conventional vs Silicone Brake Fluid – Moss Motoring
Modern Dot 4 is just fine, noting that the mandatory flush thru with fresh every couple of years is the pain in the a-- reason to change. Failure to do that, allows the water in the system…and there WILL be water from air humidity over time, to not only give a spongy brake pedal feel, but to possibly corrode wheel cylinders.
I have a complete set of internal rebuild parts for everything and will flush the whole system with alcohol. I plan on duplicating exactly what the Brady’s have done successfully in their old Jags and what they have proven to work. I went so far as to purchase the same brand of rubber parts John Brady used when he made the leap well over a decade ago.
Can you imagine going 15 years without bleeding your brakes or worrying about internal corrosion freezing up wheel cylinders and brake lines? What a luxury that would be for an investment of a only a single weekend.
Don’t get me wrong about one thing. I enjoy working in cars, but would like most of that work to be discretionary.
I wish that were possible with the cooling system as well.