Brakes sticking on

[Tony]

My brakes have started to stick on. After a bit of cooling down they eventually release. Looking through the forum it talks about the reaction valve sticking or a hydraulic problem.
Looking through various exploding diagrams I cannot spot where the reaction valve is and how to get to it and fix it.
Any good exploded drawing around showing the valve position and how to check it?

Thank you in anticipation

Tony

[mgcjag]

Hi Tony..you have a 3.8?....reaction valve is on the end of the master cylinder on 4.2 cars....I'm not clued up on the 3.8 brake system....service manual is quite detailed on the brake sections...I'm sure someone who knows 3.8 brakes will chip in..Steve

[Tony]

Thanks Steve, I will look at the manual and see what come up.

Tony

[mxke]

Have you replaced your master cylinder?
I put a new one on my 62, and would get the same symptoms. Turns out the rod was too long and when the system was warm, the master could not release due to the too long rod. Had to trim the rod down…

[Tony]

Hi mxke, no the master cylinder is an existing one and has worked fine with no problems untill today. Is stuck once which released after cooling down and a second time after about 30miles. It again release after cooling down for about 10 minutes. I managed to get home and now have to start looking into the problem. There is a lot of talk about the reaction valve sticking but I am not sure where it is and what I have to do to get it out to have a look at it. Any advice or exploded drawing s of the valve would be appreciated.

Tony. 1962 S1 3.8 convertable.

[mxke]

Check you have clearance on the rod. When it is hot it should be “loose”. If not, adjust the rod so it has a bit of slack. My front brakes would lock up after a short drive, then release when cooled off. All the issue was in that adjustment.

[mxke]

The adjustment is on the rear cylinder. If it is too long it throws the front out…

[nichmoss]

Have a look at the emergency/handbrake as well. If this sticks slightly, often caused by bent retraction forks, then it heats the rear discs which heat the rear brake circuit fluid and can cause the brakes to stick on as the fluid expands and takes up the allowance in the master cylinder rods - front and rear brakes then stick on.

See how hot the rear discs are after a short run and this will give an indication - don't touch them obviously as they can be very hot. If you're lucky, you have an inspection hatch above the diff and can see the brakes there.

Good luck, Chris

[Allrand]

Yes I had the same problem with my 3.8 after rebuild. The reaction valve sits inside the mc at the front end. The rod was too long, solved it by putting 2 washers behind the mc flange, which shifted the mc forward by 1/8". Rather than removing it & spilling brake fluid on the paint (again).

[Tony]

Hi All, thanks for all the advice. I hope to start looking at the problem starting with matching the photo provided with what is on my car. I presume the photo is from the inside of the car in the footwell. Cannot see the packing washers you mention in the photo so not sure where you have put them. As the car brake set up has been in the car for the last 15 years or more working fine then I cannot see the length of the rod being the problem. The sticking rear brake linkage could be an option but again it has been working fine for the same period. People refer to the reactor valve but they maybe talking about a later series car and not the old bellows system on a series 1. However start looking I must and will keep you updated.

Tony

[mgcjag]

Hi Tony...there is a very good Kelsey Hayes overhall artixal 4 parts in the knowledge base section...Steve viewtopic.php?f=16&t=700

[Allrand]

Hi Tony, Mike's picture is from the engine bay looking down over the mc's & pedal box, on a lhd. Here's mine, same view but obscured by the plenum on a rhd, showing the washers inserted (circled)

[Tony]

Thanks guys, I now have the air box off and can see clearly the mbc’s. I have checked under the car and all wheels are spinning freely. I am going to start by changing the brake fluid as it is old and well overdue a change. I will then have to wait and see if they lock on again. It is hard with an intermittent problem to solve anything. Time will tell.

[Allrand]

Good luck with that, I don't think changing brake fluid will solve your problem. If the reaction valve is too close to the front of the mc heat expansion of the fluid shuts it off, preventing fluid from escaping back to the reservoir. Check that your pedal is able to retract fully, if it's held even slightly depressed it will cause your problem.

[mgcjag]

Hi Randall...you've mentioned reaction valve a couple of times....I can't see a reference to "reaction valve" in the 3.8 brake section....I'm assuming your refering to the part shown as "valve" in the master cylinder drawing........Reaction valve is the name of a part in 4.2 brake systems....Steve

[Allrand]

Correct, it is a valve that closes (reacts) when pressure is applied to the pedal.

[Tony]

Ok looking at the master cylinder exploded drawing I am assuming the reaction valve you refer to is the valve at the end of the cylinder in the drawing. Before bleeding the system I will check the clearance of 1/16 inch to see if that solves the problem of the piston not fully retracting causing a build up of pressure in the system. Keep you updated on progress.

[Allrand]

Yes, it could also be that the rubber of that valve has expanded due to contamination, which will cause it to block the fluid return path. It's exacerbated by further heat build-up so the brakes can't release until the fluid has cooled.

[Tony]

Ok I have adjusted the piston rod to leave a 1/16 gap and so far that has worked on a 6 mile run. Decided to change the brake fluid as it was 13 years old . Had a problem with the remote bleed nipples not sealing at the rear. Replaced the remote bleed pipes with new and that solved the issue. So the system is now flushed and refilled with new fluid. The brakes seem to work up and down the drive, so now need to take it for a longer run to see if It has solved the original problem.
Fingers crossed. Will update you but thank you all for the advice.

I will get to the Wheels up North weekend all being well.

[Tony]

Does anybody know where I can get the fitting that goes into the rear of the brake master cylinder from the brake reservoir. What is the thread size or reference number. Are they an imperial fitting. As there is nobody local to me that make up brake pipes lines etc I need to order on line but do not know the size or thread etc.
I can get a them from SNG but at £35 a shot each it seem a bit expensive.