Brake pressure issues

Having a continual problem with the brakes. The brake pedal initially works as you would expect with pressure in the pedal and then after a drive it suddenly loses all pressure. The brakes still work if you press the brake down right to the end of travel. If you pump the pedal it comes back initially then happens again. On the way back after driving for 5 minutes they started working again normally for the entire drive home. The servo and master cylinders are new. Any ideas?

What Master cylinder do you have?
Has either reservoir fluid level been dropping? ie any leaks

My first guess is that the seal on the piston in the brake master cylinder may be deforming as engine compartment heat builds up from engine operation. If this seal does not do its job, you can push the brake pedal to the end without any brake action in that circuit and at the end, the emergency plunger pushes the little plate which sends vacuum to the booster which then actuates the other brake circuit.

Dennis 69 OTS

Begin by checking all the vacuum hoses. Give each clamp an extra turn.

It sounds to me like there is air in the system. Problem is it sounds like the air is there on occasion, and then it is not, which is what is so confusing. At this point I would say air or master cylinder, try re-bleeding to see if that solves or at least helps.

When you press the pedal, if the blue arrowed seal is defective and warping under heat and not sealing properly, instead of forcing the fluid forward and down into the booster, the fluid goes past the seal back in the direction of the reservoir. Net effect of pressing pedal is zero.
Has your reservoir fluid levels dropped at all?

69 OTS

I have had this happen…similar symptoms…one or more caliper seals had an air leak…hard to track down…but that is what it was…by the way…some calipers…usually lip seal…are prone to seal failure if the car sits…without brae use for months…it is random…may or may not happen, and not common…but it happens. The fix of course is new or rebuilt calipers…your choice if just the faulty ones…or in pairs or all 4. Nic

Air in the lines is the more likely explanation. I really think you need to check the finish work on the new master booster…check the vacuum connections, check the fluid connections, rebleed. Concentrate on the front circuit, that’s more likely where the problem is.

All great suggestions
Did you bleed the master as well?

air in lines…as a major suspect…but must consider the “how and where” did the air enter…one such if it repeats after a bleed…or won’t clear up…is as I mentioned caliper seals. Nic

One possible explanation would be water in the fluid. Under hard braking, it tends to vaporize. After cooling, it condenses and remixes with the fluid.

in any case…no matter what found…a flush thru…so that all new bra brake fluid is a fine plan, as the DOT 3-4 fluid does attract and collect water over time…thus the reason for periodic replacement. The "LMA’ that was on Castrol…meant…Low Moisture Attraction.

Its beyond me and I have limited time. I think it best to send back to the garage who did the work in the first place. I’ll let you know how I get on and thank you for all your suggestions which I have “helpfully” given to the garage!