Buick35 Posted February 21, 2018 Share Posted February 21, 2018 Hi, I couldn't get into the MGB forum so maybe some one on this site can help.I took my 74 mgb out for lunch today and on the way back home I smelled something hot and it turned out that my front disc brakes on both sides locked up. I finally got it home and after it cooled off it now rolls as it should.Last week I noticed that the brake fluid was a little low so I topped it off using synthetic fluid as a sticker on the mc said. I know I didn't overfill it. I did notice a little speck of something that could've blocked the vent hole on the cap and the rubber cap gasket was flat in a couple of areas. I guess I'll just drive it again and see what happens. Any suggestions? Thanks,Greg. Link to comment Share on other sites More sharing options...
keiser31 Posted February 21, 2018 Share Posted February 21, 2018 I would bleed the system. Link to comment Share on other sites More sharing options...
Wheelmang Posted February 21, 2018 Share Posted February 21, 2018 Not sure how a MGB compares to an MGA but you may want to check your flexible line before the distribution block to make sure it has not collapsed. Especially if it is original and you have not driven the car for a while. Link to comment Share on other sites More sharing options...
DonMicheletti Posted February 21, 2018 Share Posted February 21, 2018 If I remeber correctly the garbage British brake elastomers swell up with synthetic fluid. That can cause the problems you are having. I found out the hard way. Link to comment Share on other sites More sharing options...
1912Staver Posted February 21, 2018 Share Posted February 21, 2018 (edited) The master cylinder pushrod needs a little free play . If anything prevents the MC piston from returning to the very start of the bore { right against the retaining circlip }there is a chance the relief port in the cylinder bore will not be uncovered and the pressure in the system will not be able to vent to the fluid reservoir. A build up of "grunge" at the start of the bore will sometimes cause this. If only one wheel is locking a collapsed flex hose may be the cause, however it is unlikely both flex hoses have failed simultaneously causing both wheels to lock. P.S. I agree with Don in the above post in that British brake system rubber parts are sometimes sensitive to fluid type. However in my experience the typical result is loss of braking. Greg in Canada Edited February 21, 2018 by 1912Staver (see edit history) 1 Link to comment Share on other sites More sharing options...
Vila Posted February 21, 2018 Share Posted February 21, 2018 Not sure about a 74 MGB, but when I had my 71 Triumph TR6 back in the 1980s the brakes locked up and it turned out the Brake Servo Unit went bad. After I replaced the Brake Servo the brakes worked fine. Link to comment Share on other sites More sharing options...
CarlLaFong Posted February 21, 2018 Share Posted February 21, 2018 We always told our customers to use Girling fluid only. Other fluids could and did ruin the rubber parts in the system Link to comment Share on other sites More sharing options...
Marty Roth Posted February 22, 2018 Share Posted February 22, 2018 Flexible brake hose is actually a hose within a hose. When the inner hose fails it may create a bubble internal to the hose which may act as a "CHECK VALVE", not allowing brake fluid to return to the reservoir. Pressure will gradually bleed off, freeing the brake to allow wheel movement, only to happen again at some later time. Replace all soft lines with known quality new (Not New Old Stock) hoses, and if you are rebuilding or replacing the master cylinder and wheel cylinders, flush all steel lines with alcahol, allow to air-dry (not with air pressure), and use DOT-5 Synthetic brake fluid which is non-moisture absorbant. If not doing a total rebuild, at least add, or switch to DOT-4 which is better than DOT-3 at not absorbing moisture from the atmosphere. 2 Link to comment Share on other sites More sharing options...
Bush Mechanic Posted February 22, 2018 Share Posted February 22, 2018 (edited) If it is both discs locking on, the master cylinder piston not returning past the fluid inlet is the most likely culprit, as mentioned by 1912 Staver. I would also check the plastic switch for leakage on the brake circuit switch block. I have had trouble with the cups on the little spool valve failing. When they fail, fluid leaks out through the switch. The cups don't seem to be available, but it is not difficult to turn up another spool to accept 'O' rings, which I have done on 2 cars. Is it possible that silicone and dot 4 fluids have been mixed in the system? Not sure what reaction could be expected. I have just now mixed equal parts of the 2 fluids in a container in the workshop, as an experiment. No immediate reaction was visible, but I will monitor it over time. It could possibly create a gel, which wouldn't be ideal! Edited February 22, 2018 by Bush Mechanic cogitation (see edit history) Link to comment Share on other sites More sharing options...
Digger914 Posted February 22, 2018 Share Posted February 22, 2018 4 hours ago, Buick35 said: Hi, I couldn't get into the MGB forum so maybe some one on this site can help.I took my 74 mgb out for lunch today and on the way back home I smelled something hot and it turned out that my front disc brakes on both sides locked up. I finally got it home and after it cooled off it now rolls as it should.Last week I noticed that the brake fluid was a little low so I topped it off using synthetic fluid as a sticker on the mc said. I know I didn't overfill it. I did notice a little speck of something that could've blocked the vent hole on the cap and the rubber cap gasket was flat in a couple of areas. I guess I'll just drive it again and see what happens. Any suggestions? Thanks,Greg. The 74 MGB was built to take and expects to have DOT 3. It will work as well with DOT 4. and because it is compatible you can even add DOT 4 to the DOT 3 without causing any problems in the system. DOT 5 synthetic works in your MG after you thoroughly clean the entire brake system removing all the DOT 3, and you have verified that calipers, master and rear cylinders aren't worn and your seals are tight. When your MG was designed DOT 5 didn't exist and MG parts on the tight side of tolerance with good seals work as expected, on the loose side of tolerance they tend to leak, even when they sit. You should replace flex lines and seals when converting to DOT 5, as cleaning doesn't always get them brand new clean and adding DOT 5 synthetic to DOT 3 or 4 can cause problems, even the one described here. With a sticker on the master cylinder I would guess that it was converted at sometime with a flush and fill. Now you get to do the job that should have been done. As I looked at every little part, I know these things didn't come from the factory with a synthetic sticker on anything under the hood little part. Link to comment Share on other sites More sharing options...
Bloo Posted February 22, 2018 Share Posted February 22, 2018 (edited) 4 hours ago, Buick35 said: I topped it off using synthetic fluid as a sticker on the mc said. 3 hours ago, DonMicheletti said: If I remeber correctly the garbage British brake elastomers swell up with synthetic fluid. That can cause the problems you are having. I found out the hard way. 1 hour ago, Marty Roth said: and use DOT-5 Synthetic brake fluid which is non-moisture absorbant. 1 hour ago, Bush Mechanic said: Is it possible that silicone and dot 4 fluids have been mixed in the system? What exactly did that sticker on the master cylinder say? Traditional DOT3 and DOT4 are made from glycol, which is a synthetic product. In very recent times, the manufacturers have been labeling it synthetic. They did not do so in the past. This is causing massive confusion (in this thread too!) with DOT5 fluid, which is silicone. It is incredibly difficult to figure out for sure from manufacturers websites if the formula of DOT3 and DOT4 has changed, but it apparently has not. Just to create even more confusion, there is now DOT5.1 fluid, that is a glycol fluid like DOT3 and DOT4. It has anti-boil performance similar to DOT5 silicone, but is not chemically similar. Long ago, the British seals were rumored to be incompatible with DOT5 silicone fluid. This was probably true at the time. I doubt it is true now, but who knows how many old seals could still be in circulation. I wonder, like Bush Mechanic, if the two types of fluid got mixed. This is inadvisable, but wont really hurt anything in most cars. On the other hand, maybe the seals are old and incompatible with the silicone. Edited February 22, 2018 by Bloo (see edit history) Link to comment Share on other sites More sharing options...
Buick35 Posted February 22, 2018 Author Share Posted February 22, 2018 I'm not sure why but the shop I bought the car from had a label maker sticker on the master cylinder that states use synthetic fluid only so that's what I added. I hope it didn't scrrew things up. Link to comment Share on other sites More sharing options...
Buick35 Posted February 22, 2018 Author Share Posted February 22, 2018 I double checked the labor on the master cylinder and it read silicone not synthetic brake fluid and I topped it off with synthetic not silicone. Could that cause a problem? Thanks, Greg. Link to comment Share on other sites More sharing options...
Digger914 Posted February 22, 2018 Share Posted February 22, 2018 (edited) 1 minute ago, Buick35 said: I double checked the labor on the master cylinder and it read silicone not synthetic brake fluid and I topped it off with synthetic not silicone. Could that cause a problem? Thanks, Greg. Yep But nothing that can't be fixed with a good clean and flush. Edited February 22, 2018 by Digger914 (see edit history) Link to comment Share on other sites More sharing options...
intimeold Posted February 22, 2018 Share Posted February 22, 2018 On mine, I found the calipers seized. The car sat for a few years, in a heated building; then problems when we applied the brakes; they would not release. The calipers, were indeed seized. Wheels rolled until we applied the brakes. We had pulled it forward, out of it's parking spot in the build, rolled free; until that first application of the brakes. 1 Link to comment Share on other sites More sharing options...
DonMicheletti Posted February 22, 2018 Share Posted February 22, 2018 When I rebuilt Dunlop calipers a few years ago, I was warned not to use DOT 5 because the rubber would swell. I did a test soaking just the piston seal in DOT 5. I measured it and weighed it before and after soaking. It had gained weight and swelled considerably after a week Link to comment Share on other sites More sharing options...
capngrog Posted February 22, 2018 Share Posted February 22, 2018 18 hours ago, Buick35 said: Hi, I couldn't get into the MGB forum so maybe some one on this site can help.I took my 74 mgb out for lunch today and on the way back home I smelled something hot and it turned out that my front disc brakes on both sides locked up. I finally got it home and after it cooled off it now rolls as it should.Last week I noticed that the brake fluid was a little low so I topped it off using synthetic fluid as a sticker on the mc said. I know I didn't overfill it. I did notice a little speck of something that could've blocked the vent hole on the cap and the rubber cap gasket was flat in a couple of areas. I guess I'll just drive it again and see what happens. Any suggestions? Thanks,Greg. Buick35; There is a lot of really good information on this thread and a bit of information that is somewhat confusing ... to me anyway. I found the below article on the internet. Although a bit lengthy and technical, it appears to be well-documented and complete. The link to the article: http://www.mossmotoring.com/brake-fluid-for-classic-british-cars/ Good luck. Cheers, Grog Link to comment Share on other sites More sharing options...
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