Alternative to Jag-spec (sticking) brake master cylinder?

E-Type
1

S1.5 OTS. Wondering if there are any alternatives to continuing to replace the sticking reaction valve in my master cylinder, which I’ve now done twice. Brand new parts, usual sources.

Symptoms probably familiar to some/many - city traffic, lots of stop/go with (very) gentle prods of the brake pedal, pedal gets higher and harder until it begins to take a few seconds to release, but in my case it gets to the point where it won’t release at all.

And always at a busy stop light, usually a left turn (in US) with a line of traffic behind me, for maximum embarrassment.

I fitted a cheeky little release valve whereby I can manually bleed the vacuum from inside the cockpit, and that mostly mitigates the issue quite well, but yesterday it got to the point where even at “full bleed” it couldn’t release it quick enough and I had to pull out of the line of traffic was in and hide behind a bush - one I’ve used before for the same purpose - while I switched off and sat there for 15 minutes (!) until it finally let go.

Happily I was only 200 yards from my house and I managed to get it home, although forcibly driving it over the brakes after about half way, until I stopped it again in my gate - to the great amusement of my neighbours.

As well as the scenario above it’s also begun to happen at freeway speeds, gently braking - I always brake gently now! - to shed some speed before letting off to continue, say dropping from 70 - 50. Initially you don’t notice the drag because of the momentum of the car, but then after a few seconds the car noticeably lightens up.

At this point it’s practically undrivable for anything but the shortest distances, and my kid is 8 next week and so (in CA) is allowed to ride in the front seat and he’s begging me to pick him from school in it. Which I can’t, because it will lock up in the pickup line, and while everyone else is laughing at me he will be crying.

All I’m getting from the usual suppliers is that yes, they are seeing problems with the part overall.

There is talk of fitting a beefier spring (thoughts?) but I’m also wondering if anyone has found a part from a.n.other vehicle that fits, and works?

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There is much written on this in the archives so try a search. However I just wonder if your brake pedal and master cylinder are set up correctly with the necessary clearance when the pedal is at rest?

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Just a random thought, is it heat related? Would a blast with a product like ‘Freeze Off’ sprayed at the reaction valve speed up the release?

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Couldn’t find anything definitive, or recent, so started a new thread.

Everything is aligned correctly, pedal is fine initially.

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Yes, I think it is heat related, it primarily starts after prolonged driving in slow traffic, and particularly on hot days.

That doesn’t really apply to it happening recently at freeway speeds though.

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What are you replacing? What are you calling the reaction valve? I call the reaction valve the plastic assembly with all the innards, meaning the diaphragm, the spring, the poppet etc., not a single part. However, many issues with the reaction valve are actually traced to the small piston in the master cylinder. You have not mentioned this, so I will ask, have you replaced the master cylinder, or removed and cleaned/polished the small piston?

And Phil asked about the pedal set up. I assume he is referring to the brake pedal free play. Do you have it set to spec? And have you confirmed that when hot, the MC piston is returning fully to its stop?
Tom

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I’ve often wondered what the effect would be with a stonger spring. It would be worth while trying that IMO. The only issue is the cap that holds the spring is a snap fit on the base so I would get a good plastic glue and glue it on if the heavy spring works better.

As Tom says it’s critical that your brake pedal is able to go fully back. You should be able to move the pedal, with your hand, backwards by 1/16 to 1/8 of an inch. If it doesn’t go fully back the piston blocks the port from the reservoir

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Terry, we have discussed the spring before, it is mixed in with this thread:
Reaction valve issue - E-Type - Jag-lovers Forums

In the thread I discuss why I doubt a stronger spring would help. However, it would not be hard to try.
Tom

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I have never experienced my Series 1.5 brakes locking up lo these past 42 years but I did have a problem with brakes locking up with my late XK120 when I finally got it back on the road. Drove fine for several miles then the pedal got rock hard and the car came to a standstill. After 15 minutes or so I could continue on, until it happened again. Yes, the two tandem master cylinders are different in design and function but perhaps there’s a common element. The cure in the case of the 120 was to add just a little more freeplay between the end of the pushrod and the m/c piston. The manual calls for 1/32”, which I dutifully applied initially, but it wasn’t enough. As the brake fluid heated up it pushed the m/c piston firmly against the end of the pushrod, locking it in place and preventing the expanded fluid from flowing back up into the reservoir. I added another 1/32” of freeplay and cured the problem. That’s all it took. The hydraulics are a closed circuit when pedal pressure is applied but needs to reopen when the pedal is released. It might be worthwhile to back off your pushrod a touch, if only just to eliminate it as the issue.

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Hi Rob,

See blue arrow in photo.

I have had the problem before and in my view it is either the reaction valve piston bore, (that is inside the master cylinder), is slightly smaller than it should be, or the quality of the reaction valve piston seal rubbers (two of them) (on piston arrowed) is poor (and they swell).

Have you had this problem on two different master cylinder bodies or are you using the same body as before?

Dennis

Screenshot 2024-03-01 at 5.15.11 PM
Screenshot 2024-03-01 at 5.15.11 PM1074×922 81.6 KB

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Had the same issue on 2 completely different, new, master cylinders.

I can’t really see it can be any other part of the braking system, it’s bizarre.

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One thing you might check is to see if your car has the lower heat shield between the booster and the exhaust. The heat from the exhaust will cause problems. This is often left off on restorations for some reason.

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“Sticking brakes” is not a well-defined symptom and can describe quite a few different issues.
The small piston in the MC can stick.
The large piston in the MC can stick.
The MC free play can be too little.
The servo piston can stick.
A brake hose or line can be restricted.
A caliper piston can be sticking.
Maybe others that slip my mind.
Tom

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@Dennismo

I think that the diagram you included in your post is an erroneous one which has the plastic bearings that the piston slides in back to front. The illustration in the workshop manual and other references have the wider bearing towards the mouth of the cylinder and the thinner one towards the reaction valve end. I mention this for two reasons:

  1. Just in case anyone tries to use it to rebuild a master cylinder AND

  2. It leads in to another potential cause for the sticking brakes. Several folks here have reported that the problem that has symptoms similar to those reported by the original poster was due to the plastic bearings being too tight on the piston. When the unit heated up the piston would bind in the bearings and not fully return when the brakes were released. If you search the archives you will find at least two cases where this appeared to be the cause of non-releasing brakes when the engine bay warmed up. I myself had exactly the same symptoms on my 4.2 S1, but cannot be sure what the cause was as under time pressure I simply replaced the Master Cylinder with the (expensive) SNGB one and, touch wood, this resolved the problem.

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Thanks David!