Brake Issues

[Lagnakeil]

Hello!

I am looking for some guidance and thoughts on a problem that has just occurred.

I put the Stag on the road today, and was travelling down a bypass and accelerating past other cars. I then heard a loud 'pop' like a back fire or tyre blowing. The car continued to drive ok for a while until I went to slow down for a bend. The brake pedal went to the floor but slowed down anyway. It feels like there is a brake when the pedal reaches the floor and when you pump it, the pedal does come up. Then it drops down again when you take your foot off. The master cylinder level looks ok but it is a while since I checked. It is a replacement item too.

The servo is the original 1971 servo and my thoughts are that this may have gone. What do other people think?

Kind regards,
Lagnakeil

[FrenchStag]

If the pedal sinks to the floor but you are not obviously losing brake fluid, I would say master cylinder is most likely.

[jbuckl]

The pop could have been a backfire.
If the servo diaphragm has gone it’s possible the engine will run pretty lean…(excess unmetered air)

usually braking effort is increased dramatically if the servo doesn’t work.

if the brake pedal is going to the floor it means 1 of three things:-

the system has air in it.

the fluid is escaping somewhere after the master cylinder.

the master cylinder seals are allowing fluid past and back into the servo.

bear in mind that one stoke of the master cylinder only moves a few ml of fluid so reservoir drops quite slowly over 2/3/4 applications…. Aka when watching.

[sujitroy]

With the engine off, is the brake pedal hard? If so, it could be the servo.

[richardthestag]

If the pedal sinks to the floor but you are not obviously losing brake fluid, I would say master cylinder is most likely.

I am with French Stag on this.

If fluid level is good but pedal is suddenly long for no reason then I might be suspecting the seals on the pistons between primary and secondary circuits. the fluid is circulating between the chambers in the master and not being pushed out to the corners

[Lagnakeil]

Hello!
Thankyou all. I shall look for all of these as soon as I get the car home. I havent tried the pedal with the engine off but will do that first. If fluid has leaked into the servo, should it be changed?
lagnakeil

[FrenchStag]

I had a leaky master cylinder. I just got a bit of cloth on the end of a blunt rod and wiped the inside as best I could (small access hole). There didnt seem to be much fluid and servo seems to work fine still.

The master cylinder can be tricky to remove, I think you have to move something out of the way on a RHD Stag to get it out. Dont have that problem with my LHD.

The push rod in the servo that goes into the trumpet shaped end of the master cylinder piston can become stuck. You might have to wriggle and pull to get it out, pressing the brake pedal released it for me.

[richardthestag]

the lower mount bolt on the RHD master is a fiddle to get too, not enough room to swing a spanner and if manual then the clutch master gets in the way. I use a 1/4" drive halfords socket and extension bar that is almost the length of the master. ratchet handle and bang the nut is gone!

Undo the brake lines before the master is free, they can get tight

[MartyMann]

From what you described, I would be inclined to look at your rear brakes. It sounds like adjusted may have broke has the excess travel on the brake pedal would indicate this. Also if the pedal comes up has you pump the brake pedal it would indicate the same fault.

[Lagnakeil]

Hello Martyn!
I will do that. The car was trailered back yesterday and I have just looked underneath but cannot find any signs of a leak. I will get the rear wheels off tomorrow.
lagnakeil

[MartyMann]

Hi Lagnakeil,
I don’t think it a hydraulic issue, I believe it might be mechanical in the drum brake at the rear, I might be wrong but it,s a quick thing to check before getting too involved.
Good luck

[Lagnakeil]

Hello Martyn,
I will be putting the car on a hydraulic lift this weekend to check for any leaks in the lines. From a preliminary look, I cannot see anything amiss nor any leaks. The brake pedal has come back up again and is holding as it should. So you may well be right.
If we cannot see any leaks on Saturday, I will remove the drums and check the rear brakes.
Kind regards,
Lagnakeil