Does it have a vacuum brake booster? If so, it may be that the pushrod inside the booster is just slightly pressurizing the master cylinder because of the spacing between that pushrod and the master cylinder piston rod. Did you replace the master cylinder with another one?
Or replace the booster?
Trapped air can do this as well, if the brakes are not bled completely.

 

Thank you for the response, no vacuum booster no, just the 3 line small master of which I did replace yes. Trapped air hey.....would that release as soon as I threw water on the calipers? That is what is confusing me. Brakes are great work good until they are heated and then they work REALLY GOOD.

I will re-bleed the system tonight and see if that helps. Appreciate your help @alfaloco

 

OK-- yes I had this issue with a Suzuki sport bike years ago, with twin discs up front. I traced it down to air trapped in the hoses. Kind of hard to bleed it out when the master cylinder was much higher than the calipers.
It doesn't take much of an air pocket, because as you know when you heat air, it expands. Bingo-- pressure on the pads.
Check the reservoir also to ensure it's well seated on the rubber seal where it pops into the top of the master cylinder.

 

If the push rod that connects the pedal mechanism to the master cylinder is too long the piston will not retract far enough to open the inlet port of the master cylinder. This will prevent bleed-back to the reservoir, holding pressure in the system

Don

 

If the push rod that connects the pedal mechanism to the master cylinder is too long the piston will not retract far enough to open the inlet port of the master cylinder. This will prevent bleed-back to the reservoir, holding pressure in the system
Don

My Lira is on this ☝

 

Hey Don, I did consider this but it is the same type of Master and I don't believe there is any adjustment on it.....I will have to look at that to confirm though. My quandary is would throwing water on the caliper instantly release them if it was a master issue?

 

and why just the front not the rears?

 

Will brakes can be a mystery! they have mystified me a couple of times.

 

Why just the front? Good question, is it both fronts? If so unlikely rubber hoses, are they new? If it happens again crack a rear bleed momentarily and see if that has any affect, if still locked crack a front bleed. That should give more clues, but is a pain to do.

Are steel brake lines new and clean, no debris?

Don

 

Yes both fronts.....everything is new, master, hard lines braided lines and rotors and pads. Doing some reading on a hot rod forum and there was a suggestion of maybe the master cylinder to close to the body for the push rod....wonder maybe if I throw a small washer in and see if this makes a difference? Thoughts?

 

That's easy enough to do, and worth trying IMO. Assuming there's no adjustment on the pedal travel.

 

Hey Don so it doesnt appear to have any adjustment, Are you thinking the whole rod spins? The clevis does not have any adjustment on it as per the pic here. Sorry was on my back on the driveway.

I found the pedal is extremely hard as well, very little play at the top of the pedal.....

 

There is a jam nut between bellows and clevis. Take out the cotter pin and remove the clevis pin. Unlock nut, a screwdriver will keep the part from rotating. Adjust length shorter and reassemble.

 

Ok...I think I see that.. will try.....once again appreciate the patience!

 

So sitting on the side of the road waiting for these brakes to cool down and unlock. I loosened the rod 3 full turns and Bleed the crap out of the brakes.....but after about 15 mins of easy driving they started to tighten and bam....locked

Could I have my brakes installed wrong....pin spacers?

 

Very strange, but at least you have a very clean car to work on..

I don't suppose you have a old (read thinner) set of brake pads to try. I am wondering if the pads are dragging and expanding with nowhere to go.

How far can you drive from cold without using the brakes or running in to something. ie do they lock without applying pedal effort? Your car should have a pressure switch to illuminate the rear lights. You mentioned the lights remain on so something is definitely maintaining pressure...but what


Don

 

Well Don tonight I timed it I drove for 14 mins very mellow hardly used the brakes. Then the pedal gets really hard and within a min or so they completely lock up. Brake light stays on then goes off when they cool. I can here something at the caliper almost a tick tick....tick tick its weird.

I do not have any old pads nope....the calipers were seized so I rebuilt all 4, I wonder if the pistons were not good enough to reuse and they are messed? Again why only when heated....nothing expands in there other than the fluid. Should I throw some more money at this and replace with new caliper ups front? Would eliminate those?

 

Well Don tonight I timed it I drove for 14 mins very mellow hardly used the brakes. Then the pedal gets really hard and within a min or so they completely lock up. Brake light stays on then goes off when they cool.

My 0.02 Lira's worth is your master cylinder is being boiled by the exhaust & the fluid is expanding internally & the piston is blocking the fluid return hole - you have the MC pushrod too tight (yet you said you backed it off 3 turns...)

I can here something at the caliper almost a tick tick....tick tick its weird.

Internal pressure is bleeding slowly back through the MC as it cools down, with each drop in fluid pressure the calipers un-clamp just a fraction & that's your noise

2 Simple things - do you have a loose sheet of aluminium, old license plate, neighbour's license plate etc & can you fabricate a quick & dirty cover over your MC & just wire it into place - better still if you have some heat resistant material to stick on it & then test again - if your test drive goes over your 14/15 mins to say 25 then your MC is running cooler

It's a quick & dirty test, but won't cost you any $$ except for gas & hey a drive is a drive

Crack a bleed nipple to release pressure & get home

I'd strip & re-check the MC, easy enough to do with a standing pedal car

Ciao
Greig

 

Do both front wheels lock or just one ?? One is a problem on that corner, two lead back to a common denominator the master cylinder

As Alfaloco said, when things heat up they expand, an easy way to check is to momentarily crack a bleed nipple and see if you get a spurt from both calipers, that'll instantly release them. Just crack & lock so you don't suck air.
As dbest says - how far can you drive from cold without applying brakes. In this scenario your MC is heating up from the exhaust on a LHD car.

Take a decent shop trolley jack with you on your next test and jack up each front wheel - check for drag/lock on each wheel BEFORE you crack a bleed nipple, that way you know if both or just one is dragging. can test the rears as well - take your soft wood block to lift on the diff pumpkin. if it's both fronts & rears --> it's the MC

I had this with my GTJ and it was braided hoses that were failing internally, the rubber had perished and was acting like a 1 way valve, fluid simply could not return. In my case it was easy, cracking the nipple produced a thunk from the MC as the piston returned, so I knew something was blocking the return. Bleeding the 4 corners produced clean fluid, pulling the MC revealed lots of rusty syrup, despite clean fluid being returned out the bleeders. Pushing a length of brazing rod down the flexible revealed ribbons of internal rubber being extruded out the other end.

Are your braided hoses NEW or New Old Stock (NOS) ??

You can do a crude test on hoses using a length of brazing rod - round the blunt end edges first and gently wiggle it through a loose hose.

It's going to be simple - when you find it... Ahh the joys of brakes

Ciao
Geig

 

Thanks for the reply Greg, they are brand new braided lines, steel lines replaced as well front and rear...master new. Essentially the only old stuff is the rebuilt calipers and the pressure sensor switch. It is both front that are locked rears are free that is what is confusing. I am second guessing my rebuild on the caliper now but they work so good for 12 mins!!!

 

If you jack up the front of the car so the front wheels spin freely. Then push on the brake pedal hard and slowly release. After that do your front wheels spin freely again? If so, your calipers are fine.

 

Spec on piston to push rod clearance is 1.0 mm

Don

 

Your brake hardware is fine. The bleeders are upward, which is correct. I really believe you'll find the solution via the master cylinder, or the brake hoses (but you say they are new, so unlikely they are the cause).
Take Greig's suggestion and jack up the front once the brakes are hot and dragging. Test the rotation of each wheel. Crack open the bleed fittings and see if you get a little spurt of fluid, and then the wheel loosens. If that's what you see, the master cylinder is not releasing pressure applied to the pads, or something downstream of the master cylinder (like old brake line tubing, with some crud inside??). You may have to readjust that pedal clevis again until the internal piston clears the inlet port from the reservoir.

 

Thanks guys, will play with it this weekend and see what I can figure out. Will report back. Really appreciate all the efforts Cheers

 

Biggs, I still think the solution is in the master cylinder adjustment, but something you said in post 19 concerns me also. You said the "brakes were seized", and you indicated you rebuilt all 4 calipers. It is possible that we have more than one gremlin here, if the pistons were seized in the calipers, maybe corroded, that's not going to give you consistent brake action.
Anyway... I am with the others here, adjust the master cylinder pushrod a bit at a time, see what results you get. AND check the fittings and line down from the fluid reservoir under the hood, also. Make certain they are tight and not introducing air into the system as you operate the pedal. If a hydraulic system can suck in air, it will do so, believe me.

 

What a beautiful car.
There are two types of brake master cylinders for the vacuum assisted and non vacuum assisted roundtails, single circuit. Counter intuitively, the larger one goes with the vacuum assisted system. Do you have the correct sizing?

 

Size of the master bore will not cause this issue. I have the 22mm cylinder on a system with no servo as the 20mm is not made anymore. Needs more pressure but works fine. The master rod needs to be adjusted so that there is free play in the mechanical system that moves the master rod. The pedal should move some amount before the rod moves the master. This insures that the return valve can open.

 

Hey guys may have had some break through here. So tonight I lifted the car and ran it.. back brakes worked great.. so I took it for a drive to do the above mentioned tests.. I wanted to make a certain rd which is dead and flat so I started hitting the brake more than normal. Honestly u don't really need much in way of brakes with this thing... anyway so I took it up to 4th and stood on the brake pedal to try to lock the brakes up and pedal went soft and slowly went to the floor. Pedal came back up and now everytime hit the brakes hard it goes soft and to the floor. I was not able to get them to sieze at all now.

So I am assuming I have air in the master...none of the lines are leaking so I can't be sucking in air. Wdt?

 

Fluid leak past the washers in the master cylinder - you are loosing pressure in the main hydraulic pump, which is the M/C. You said you got a new master cylinder - did you just fit it, or did you first strip, clean, check, re-assemble & then fit ?? Reason I ask is because a friend got a new reconditioned (stainless sleeved) twin cylinder for his Austin Healey Sprite and had bad brakes, I went over to help, 2 hours of bleeding later I pulled the newly rebuilt M/C off and washed out the most incredible amount of machining swarf from the innards.... sold as fully assembled & ready to fit. Needless to say I fitted new washers as the supplied new ones had been running in grinding paste

We've identified the source of your issue as the MC where the piston isn't returning far enough to uncover the feed hole from the reservoir which is also the bleed-back return for excess fluid from heat expansion. In your shoes I'd take the MC off and strip it on the bench, then check carefully. You said new, but is it reconditioned with a stainless sleeve or a brand new cast iron unit. Check that the fluid feed holes have been drilled through correctly and that there are no sharp edges or burrs protruding into the smooth bore.

Brake Cleaner spray is a good start to getting things clean, Remember that the ONLY other cleaning fluid compatible with brake fluid is Methylated Spirits or White Spirit, nothing else. Use 3 clean ice cream tubs in a row, wash and clean using an old tooth brush is the first one, again in the second tub and a final rinse off in the third one. Stand to dry on clean paper towel. A dollar to a dime you will have muck in the bottom of the first two tubs. Now you need new washers as if there's swarf in the tubs the washer edges will have sustained damage.

I'm a bit of a pedant with brakes, but the alternative is dire & price of bodywork & chrome is heavy

Aye
Greig