Brakes sticking on

[peteraf]

At risk of boring you with a similar issue to the one posted recently, and having tried many of the suggestions in that post, I've run out of ideas on how to fix the problem on my 1970 P5B.

My original problem was spongy brake pedal, which I fixed by replacing the master cylinder (used original pushrod and adjuster). After bleeding the brakes I found that the front and rear brakes were locking on. Did some adjustment of the MC push rod and set the pedal height to 6 inches as per WSM. This did not fix it.

Then looked at the servo - despite this working fine previously. I found the air valve filter was missing, so replaced that. Also disconnected the vacuum hose and plugged the ends, but this did not fix it.

Thoughts then went back to the MC. Taking it off, I found that the pushrod retaining circlip was partially out of its groove, so I secured it properly and put the MC back on the car. Adjusted pedal height again but still had sticking brakes. BUT, I think there was an improvement as operating the brakes when stationary, or just shunting in and out of the garage did not cause sticking; taking it for a test drive caused the brakes to gradually stick on after about 4-5 minutes. The front brakes stick more than the rear - possibly due to the rear adjusters being backed off a bit.

I have found that the brakes ease off a bit slowly after a while, enough to be able to push the car.

I'm wondering if the MC is working properly, despite being brand new (from Roverparts: described as "Lucas Girling).

Appreciate thoughts and suggestions.

regards
Peter

[Phil Nottingham]

Seems like the MC - is repo or NOS?  

[enigmas]

Apr 26, 2024 13:09:48 GMT peteraf said:

At risk of boring you with a similar issue to the one posted recently, and having tried many of the suggestions in that post, I've run out of ideas on how to fix the problem on my 1970 P5B.

My original problem was spongy brake pedal, which I fixed by replacing the master cylinder (used original pushrod and adjuster). After bleeding the brakes I found that the front and rear brakes were locking on. Did some adjustment of the MC push rod and set the pedal height to 6 inches as per WSM. This did not fix it.

Then looked at the servo - despite this working fine previously. I found the air valve filter was missing, so replaced that. Also disconnected the vacuum hose and plugged the ends, but this did not fix it.

Thoughts then went back to the MC. Taking it off, I found that the pushrod retaining circlip was partially out of its groove, so I secured it properly and put the MC back on the car. Adjusted pedal height again but still had sticking brakes. BUT, I think there was an improvement as operating the brakes when stationary, or just shunting in and out of the garage did not cause sticking; taking it for a test drive caused the brakes to gradually stick on after about 4-5 minutes. The front brakes stick more than the rear - possibly due to the rear adjusters being backed off a bit.

I have found that the brakes ease off a bit slowly after a while, enough to be able to push the car.

I'm wondering if the MC is working properly, despite being brand new (from Roverparts: described as "Lucas Girling).

Appreciate thoughts and suggestions.

regards
Peter

You've seemingly changed quite a few factory fixed adjustments whilst attempting to sort the binding brake issues. Usually when brakes lock on and won't release it can be traced to a power brake servo issue, but as you've disconnected and plugged the vacuum line to the servo (as a test) that can be discounted.

You've also played with the master cyl pushrod adjustment, setting it to the WSMs 6" pedal height.
For the moment forget the pedal height and ensure that the master cylinder pushrod is not engaging the master cylinder hydraulic piston assembly when the brake pedal is at rest.

Note: There must be some "free play" between the brake pedal pushrod and the primary hydraulic piston of the master cylinder or the recouperating ports will be blocked...fluid is then unable to return to the master cylinder when pedal pressure is released.

The degree of free play is minimal but the pushrod must not engage the hydraulic piston at rest or at pedal release.

[peteraf]

Phil - bought from Roverpart; described as "BRAND NEW GENUINE GIRLING / LUCAS PART".

Enigmas - I checked for free play on the pushrod. So difficult to tell in situ. I disconnected the pushrod adjuster from the brake pedal and pushed it back and forwards but did not detect any free play. Presumably a slight play should be felt? I tried pulling on the adjuster but still could not detect any play. Does that point to an issue inside the MC?

Peter

[lagain]

It depends on how old the master cylinder is. The chances are that it is not brand new and that the seal is sticking. Unfortunately it means taking it off and checking it. When disconnected the plunger should slide smoothly down and return on its own. If you do need new seals, get them from Wadhams, as he has the largest turnover. Any brake part with unknown age of rubbers needs redoing.

[Phil Nottingham]

Apr 26, 2024 15:56:56 GMT peteraf said:

Phil - bought from Roverpart; described as "BRAND NEW GENUINE GIRLING / LUCAS PART".

Enigmas - I checked for free play on the pushrod. So difficult to tell in situ. I disconnected the pushrod adjuster from the brake pedal and pushed it back and forwards but did not detect any free play. Presumably a slight play should be felt? I tried pulling on the adjuster but still could not detect any play. Does that point to an issue inside the MC?

Peter

I doubt brand new Lucas would be any better quality or UK made than repo modern parts from Rover suppliers.  NOS Lucas will be great but the seals etc may very well have deteriorated in storage by now.

Your problems do point to an internal master cylinder fault unless the free play is still not sufficient - try more.

Is the fluid cap air vent clear?

[enigmas]

Apr 26, 2024 15:56:56 GMT peteraf said:

Phil - bought from Roverpart; described as "BRAND NEW GENUINE GIRLING / LUCAS PART".

Enigmas - I checked for free play on the pushrod. So difficult to tell in situ. I disconnected the pushrod adjuster from the brake pedal and pushed it back and forwards but did not detect any free play. Presumably a slight play should be felt? I tried pulling on the adjuster but still could not detect any play. Does that point to an issue inside the MC?

Peter

Peter just ensure that the master cylinder pushrod has "absolutely no load on it" when it is fully retracted. The hydraulic piston seal assembly when at rest will be pushed back fully by a spring within the cylinder.The push rod in this situation is fully retracted.

When you "now" attach the pedal to the pushrod ensure that it places no load on the pushrod. This is what I'm referring to as freeplay...between the pedal and the pushrod.

Ensure that there is not a spring pulling the pedal down and imposing load on the pushrod. If so, disconnect or alter it. For the moment, discount the 6" pedal adjustment or whatever the WSM states.

If after doing the above the sticking brake issue still exists, then the issue is within the master cylinder. Recheck the component assembly and the cup seals, etc.


*** One last point of contention that may be causing the issue are the brake hydraulic hoses. After long use the internal liner of the OEM rubber style hydraulic hoses clog up. Under high hydraulic pressure fluid moves through to the caliper (disc brake) or wheel cylinder if a drum brake. When the pedal is released fluid is held back as it is not under high pressure. This situation will cause dragging brakes.

Here is a picture of a blocked hydraulic hose that I removed from one of my classics.
Note: The thin TIG welding wire will not pass through the inner brake hose liner.


Nor could fluid be released from the caliper assembly via the hose when pressure is applied to retract the caliper piston using the clamp as shown.

[roverp5b]

My memory on this is very vague now but I had this similar problem for years changed all sorts and then one day a mechanic noticed the brake light switch was adjusted in a way that never let the brake pedal fully release, as I said it’s a vague memory but no harm checking.

[Phil Nottingham]

The flexis can cause locking of brakes but generally only on the one wheel or axle not all. New flexis do not last as long as the old ones either.

The master cylinder looks to be the culprit but there could be a restriction/blockage in the pipe from the servo to the T piece?

The system is very simple even with its simple Servo as such there are few places that can cause this

[peteraf]

Thanks all for the suggestions.

A bit of an update on latest checks:

- Overnight the brakes have freed - nice spinning wheels.
- I've got the clear P6 type reservoir. Apart from a fluid soaked cork it looks OK, but I can't see the vent hole - is it via the spindle and contact cap?
- Fiddled with the pushrod adjuster again. After pressing the brake pedal a few times the brakes locked up; tried to pull out the pushrod to give free play but this did not release the brakes. Screwed the adjuster in a bit so that the pedal now sits lower, off the brake light switch, with the idea that the spring tension pulling the pedal up should ensure the pushrod is pulled out from the MC as much as possible. Brakes still locked!
- Appreciate that flexi hoses / pipe blockages could cause the problem, but these have not been disturbed and given all four wheels lock up seems unlikely?


So, looks like next step is to take the MC off again and check it thoroughly.

[lagain]

The push rod is not attached to anything, it pushes the seal in, but cannot pull it back out, the internal spring pushes the seal back. If the seal is sticking, with the rod detached from the pedal there will be a little movement, but I still think that it would be a good idea to remove the master cylinder and check that the piston is sliding smoothly and easily in the bore.

[Phil Nottingham]

The plastic ones vent through the switch cap terminal holes BUT the spindle the cork floats on can become bunged up thus blocking air flow. The easiest way to check this out is to just leave the cap off and try the brakes.

As George pointed out the m/c piston returns FULLY by the cylinder internal spring - the pedal return spring just stops it coming out fully if its push rod is not adjusted to give free play - it clearly is probably fully not returning either because of incorrect internal assembly or sticky ridge/bore.  The distance between the piston returning past the inflow hole from the reservoir is very small. Perhaps debris is in the bore or recovery drilling that needs clearing?

[peteraf]

Problem solved.

Took off the MC and found that the cupped washer that retains the pushrod was the wrong way round. Turning it around enabled the pushrod to have free play to the piston. See photo for correct position.

I obviously assembled it incorrectly when fitting the old pushrod and washer onto the new MC. Doh! Lesson learned: always record details when taking things apart.

Thanks all for your suggestions.

Peter