Silly question, did you open the radiator bleed valves while you were draining the heating system? If not, you have probably drained the small amount of water from your CH F & E tank and then caused an air lock, which is disturbed as you try to open the rad valve joints.
If the above isn't the case, make sure the bleed valve on the bathroom radiator isn't blocked- remove the valve and put a long pin inside. If air can't get in the radiator, water can be held there.
Even if the radiator is off the primary circuit to your hot water cylinder, it will be an indirect cylinder, therefore, it will still drain when you drain the heating. The only time it wont drain properly is if you have a hole in the primary circuit and water is being forced in to your heating system from the Hot Water F & E tank. If you leave your stop tap on but isolate the ball valve to your CH F & E tank by holding the ball valve in the up position with string or similar, then open the drain valve on the heating system, if the ball valve inside the Hot Water F & E tank begins passing water, then you have a breach.
If the primary is breached then you will need to drain the cylinder as well- which should also be replaced, otherwise you will get contaminants from the heating system in your hot water.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!