Seizing brakes

[HQentity]

I've had this problem with various cars over the years, the brakes gradually applying themselves till the car becomes undriveably locked stationary with the pedal rock hard and no free travel. The last time was last week on a customers Sprint! But it is particularly prevalent amongst the Peugeot/Citroen/Fiat family of vehicles. This is nearly ALWAYS down to the servo!

A different test for you, when the car is locked stationary, DO NOT release hydraulic pressure at the pipe, instead remove the stiffener bolt on the turret and loosen the master cylinder to servo nuts to the end of the threads, pull the master cylinder forward as far as possible, then see if the car will roll from pushing it! This will determine if the problem lies with the servo (car CAN be pushed with master cylinder bolts loosened) or hydraulic system (car CANNOT be pushed with m/cyl bolts loosened)

Now I don't know, nor do I know anyone who DOES, what causes a servo to misbehave in this way. Since a second hand servo is almost always cheaper than even a rebuild kit (if you can even get one) I simply replace the servo, job done! And i'm not sufficiently curious to dismantle the old one to see if I can see what's amiss, it just goes in the scrap pile with the rest of the rubbish!

Steve

Will doing this not let the brake fluid escape between the master cylinder and servo? Am I imagining this correctly?

[Carledo]

I've had this problem with various cars over the years, the brakes gradually applying themselves till the car becomes undriveably locked stationary with the pedal rock hard and no free travel. The last time was last week on a customers Sprint! But it is particularly prevalent amongst the Peugeot/Citroen/Fiat family of vehicles. This is nearly ALWAYS down to the servo!

A different test for you, when the car is locked stationary, DO NOT release hydraulic pressure at the pipe, instead remove the stiffener bolt on the turret and loosen the master cylinder to servo nuts to the end of the threads, pull the master cylinder forward as far as possible, then see if the car will roll from pushing it! This will determine if the problem lies with the servo (car CAN be pushed with master cylinder bolts loosened) or hydraulic system (car CANNOT be pushed with m/cyl bolts loosened)

Now I don't know, nor do I know anyone who DOES, what causes a servo to misbehave in this way. Since a second hand servo is almost always cheaper than even a rebuild kit (if you can even get one) I simply replace the servo, job done! And i'm not sufficiently curious to dismantle the old one to see if I can see what's amiss, it just goes in the scrap pile with the rest of the rubbish!

Steve

Will doing this not let the brake fluid escape between the master cylinder and servo? Am I imagining this correctly?

NO! If fluid comes out of the gap, the master cylinder seals are fubared!

Steve

[HQentity]

ah, of course! The last time I did this, the seals were BAD bad, hence, the fluid! Let's see what happens! Back in a mo...

[HQentity]

Servo it is! Car immediately moved upon moving the master cylinder. No fluid came out.

[TahitiSPRINT]

Before replacing the servo: Does your brake pedal returns fully to its original position if released? Thus not hitting a dash cover board or an out of line brake pedal switch?

I ask this because if the brake pedal can not fully retract, then the compensation valve in the brake master cylinder will not open and pressure will stay in the brake lines eventually locking the brakes when they heat up.

Good luck!

[HQentity]

It seems to be fully retracting ok to me - sometimes recently, it has fully retracted, but the brakes would not begin to engage until I'd pressed down a cm or so? Like something had stuck somewhere. What is the opinion on refurbishment kits like this? The part number I believe I need is 519970 which is used on Quiller Triumph for £25, which seems like the best I can find it for.

[MIG Wielder]

Hi Kyle, Yes, if you find you need a complete 1850 servo that is the correct part number. The problem is that if you go for a Quiller's part, the price is £92 with the surcharge being £25.
It depends on how your browser resolves the text.
Yes, 1850 servos are getting scare.
1. Drop a P.M. to Alun to see what he can do.
He has being looking at refurb units.

2. The first thing I'd do after your masterful diagnostic checks... are ... Is the return spring O.K. ? 154834
Is the stop light switch O.K. ? Unscrew it completely to check. Its a lot cheaper than a servo.

HTH
Tony.

[HQentity]

Hiya Tony,

Thanks for the info. Gutted about the cost of the brake servo, thought I'd struck a bit of luck there! My partner Jess recommends splashing out and getting a new one anyway, although I'd much rather save for the new dining room table as we mentioned the other day!

I have messaged Alun as suggested, hopefully he'll have a solution for me, thank you!

I'm also confident the return spring is absolutely fine - the pedal always returns just fine to the highest point, & the stop light switch was replaced reasonably recently too.

Thanks for the help!

Kyle

[Carledo]

Ronald's post has made me think too!

When I had the same problem with that Sprint last week, After fitting the replacement servo, I had trouble refitting the pedal clevis pin in the servo pushrod because the brake light switch bracket was bent a tad. Firm application of a pair of water pump pliers solved this and I continued and finished the job.

But in your case, it might be worth doing this check, BEFORE you go to all the trouble (and potential expense) of pulling the servo off.

Simply remove the split pin and the pedal clevis pin, then try and refit it! If it's hard to get it back in because the pushrod is too far into the pedal for the holes to line up, give the switch bracket a tweak till it goes in easily, then try the car again! Takes only a few minutes, costs nothing and MIGHT cure the problem!

Steve

[Bumpa]

You won't want to hear this, but I have just finished refurbishing my 1850HL and after lying idle for 26 years the servo was knackered and very rusty. I sent it to Wins International and it came back after several weeks looking and working like new. The cost - gulp- £234. Well, its only money.....

[HQentity]

Ok, so I had a look at the brake pedal a little more this morning, and moved a few cables around to keep the pedal as clear as possible. I drove around my village for a good 10 minutes braking all the time and the pedal didn't rise hardly at all. It STILL ended up with that 1cm or so of free travel before engaging the brakes though.

I'm not convinced its the servo. I can pump the brakes and it doesn't always get worse. I swear it's worse when the car is hotter. I'm not convinced its not some dirt in the master cylinder jamming the piston. The test I did by removing the master cylinder maybe just freed it up by hitting it - it only came a mm or so away from the servo when the brakes freed off? Might just get a MC refurb kit and see if that solves it?

[xvivalve]

I'm sure I could get 1850 servos reconditioned for the same price as Sprint ones, maybe less, I just haven't approached my guy with them as I perceived there were plenty of those around? As you'll see in the most recent edition of DM, Sprint ones are £145.

A duff single line master cylinder caused my brakes to seize on the evening before I moved from Weybridge back to Cardiff; fortunately the son of a chap I worked with was a mechanic at Lex Weybridge and managed to source and fit a new unit for me overnight

[Magenta Auto Sprint]

Now I don't know, nor do I know anyone who DOES, what causes a servo to misbehave in this way. Since a second hand servo is almost always cheaper than even a rebuild kit (if you can even get one) I simply replace the servo, job done! And i'm not sufficiently curious to dismantle the old one to see if I can see what's amiss, it just goes in the scrap pile with the rest of the rubbish!

Steve

I had a similar problem on the sprint servo, it turns out that the pin that pushes the BMC was not adjusted as per the workshop manual, this prevents the BMC piston from returning to its correct position therefore the brakes lock on.

so I would conclude that the diaphragm in an old servo must start to stick in the rear part of the housing despite having a quite large spring in the front section. I f the diaphragm can't return then the piston the in BMC can't return either.

I can't think of anyway that this sticking can be rectified, with the servo on a bench, I'm not sure that spraying WD 40 would do the rubber diaphragm any good.

Malcolm

[AlanH]

I'm sure I could get 1850 servos reconditioned for the same price as Sprint ones, maybe less, I just haven't approached my guy with them as I perceived there were plenty of those around? As you'll see in the most recent edition of DM, Sprint ones are £145.

Will a Sprint servo go fit an 1850?

[xvivalve]

It would, but they are scarce items, which is why there is £100 surcharge on them if a serviceable unit cannot be offered in exchange.